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    SubjectRe: [PATCH v3 4/5] trace: Make removal of ring buffer pages atomic
    On Tue, Aug 23, 2011 at 11:55 AM, Vaibhav Nagarnaik
    <vnagarnaik@google.com> wrote:
    > This patch adds the capability to remove pages from a ring buffer
    > without destroying any existing data in it.
    >
    > This is done by removing the pages after the tail page. This makes sure
    > that first all the empty pages in the ring buffer are removed. If the
    > head page is one in the list of pages to be removed, then the page after
    > the removed ones is made the head page. This removes the oldest data
    > from the ring buffer and keeps the latest data around to be read.
    >
    > To do this in a non-racey manner, tracing is stopped for a very short
    > time while the pages to be removed are identified and unlinked from the
    > ring buffer. The pages are freed after the tracing is restarted to
    > minimize the time needed to stop tracing.
    >
    > The context in which the pages from the per-cpu ring buffer are removed
    > runs on the respective CPU. This minimizes the events not traced to only
    > NMI trace contexts.

    "interrupt contexts". We're not disabling interrupts.

    >
    > Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@google.com>
    > ---
    > Changelog v3-v2:
    > * Fix compile errors
    >
    >  kernel/trace/ring_buffer.c |  225 ++++++++++++++++++++++++++++++++------------
    >  kernel/trace/trace.c       |   20 +----
    >  2 files changed, 167 insertions(+), 78 deletions(-)
    >
    > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
    > index bb0ffdd..f10e439 100644
    > --- a/kernel/trace/ring_buffer.c
    > +++ b/kernel/trace/ring_buffer.c
    > @@ -23,6 +23,8 @@
    >  #include <asm/local.h>
    >  #include "trace.h"
    >
    > +static void update_pages_handler(struct work_struct *work);
    > +
    >  /*
    >  * The ring buffer header is special. We must manually up keep it.
    >  */
    > @@ -502,6 +504,8 @@ struct ring_buffer_per_cpu {
    >        /* ring buffer pages to update, > 0 to add, < 0 to remove */
    >        int                             nr_pages_to_update;
    >        struct list_head                new_pages; /* new pages to add */
    > +       struct work_struct              update_pages_work;
    > +       struct completion               update_completion;
    >  };
    >
    >  struct ring_buffer {
    > @@ -1080,6 +1084,8 @@ rb_allocate_cpu_buffer(struct ring_buffer *buffer, int nr_pages, int cpu)
    >        spin_lock_init(&cpu_buffer->reader_lock);
    >        lockdep_set_class(&cpu_buffer->reader_lock, buffer->reader_lock_key);
    >        cpu_buffer->lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
    > +       INIT_WORK(&cpu_buffer->update_pages_work, update_pages_handler);
    > +       init_completion(&cpu_buffer->update_completion);
    >
    >        bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
    >                            GFP_KERNEL, cpu_to_node(cpu));
    > @@ -1267,32 +1273,107 @@ void ring_buffer_set_clock(struct ring_buffer *buffer,
    >
    >  static void rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer);
    >
    > +static inline unsigned long rb_page_entries(struct buffer_page *bpage)
    > +{
    > +       return local_read(&bpage->entries) & RB_WRITE_MASK;
    > +}
    > +
    > +static inline unsigned long rb_page_write(struct buffer_page *bpage)
    > +{
    > +       return local_read(&bpage->write) & RB_WRITE_MASK;
    > +}
    > +
    >  static void
    > -rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned nr_pages)
    > +rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned int nr_pages)
    >  {
    > -       struct buffer_page *bpage;
    > -       struct list_head *p;
    > -       unsigned i;
    > +       unsigned int nr_removed;
    > +       int page_entries;
    > +       struct list_head *tail_page, *to_remove, *next_page;
    > +       unsigned long head_bit;
    > +       struct buffer_page *last_page, *first_page;
    > +       struct buffer_page *to_remove_page, *tmp_iter_page;
    >
    > +       head_bit = 0;
    >        spin_lock_irq(&cpu_buffer->reader_lock);
    > -       rb_head_page_deactivate(cpu_buffer);
    > -
    > -       for (i = 0; i < nr_pages; i++) {
    > -               if (RB_WARN_ON(cpu_buffer, list_empty(cpu_buffer->pages)))
    > -                       goto out;
    > -               p = cpu_buffer->pages->next;
    > -               bpage = list_entry(p, struct buffer_page, list);
    > -               list_del_init(&bpage->list);
    > -               free_buffer_page(bpage);
    > +       atomic_inc(&cpu_buffer->record_disabled);
    > +       /*
    > +        * We don't race with the readers since we have acquired the reader
    > +        * lock. We also don't race with writers after disabling recording.
    > +        * This makes it easy to figure out the first and the last page to be
    > +        * removed from the list. We remove all the pages in between including
    > +        * the first and last pages. This is done in a busy loop so that we
    > +        * lose the least number of traces.
    > +        * The pages are freed after we restart recording and unlock readers.
    > +        */
    > +       tail_page = &cpu_buffer->tail_page->list;
    > +       /*
    > +        * tail page might be on reader page, we remove the next page
    > +        * from the ring buffer
    > +        */
    > +       if (cpu_buffer->tail_page == cpu_buffer->reader_page)
    > +               tail_page = rb_list_head(tail_page->next);
    > +       to_remove = tail_page;
    > +
    > +       /* start of pages to remove */
    > +       first_page = list_entry(rb_list_head(to_remove->next),
    > +                               struct buffer_page, list);
    > +       for (nr_removed = 0; nr_removed < nr_pages; nr_removed++) {
    > +               to_remove = rb_list_head(to_remove)->next;
    > +               head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD;
    >        }
    > -       if (RB_WARN_ON(cpu_buffer, list_empty(cpu_buffer->pages)))
    > -               goto out;
    >
    > -       rb_reset_cpu(cpu_buffer);
    > -       rb_check_pages(cpu_buffer);
    > -
    > -out:
    > +       next_page = rb_list_head(to_remove)->next;
    > +       /* now we remove all pages between tail_page and next_page */
    > +       tail_page->next = (struct list_head *)((unsigned long)next_page |
    > +                                               head_bit);
    > +       next_page = rb_list_head(next_page);
    > +       next_page->prev = tail_page;
    > +       /* make sure pages points to a valid page in the ring buffer */
    > +       cpu_buffer->pages = next_page;
    > +       /* update head page */
    > +       if (head_bit)
    > +               cpu_buffer->head_page = list_entry(next_page,
    > +                                               struct buffer_page, list);
    > +       /*
    > +        * change read pointer to make sure any read iterators reset
    > +        * themselves
    > +        */
    > +       cpu_buffer->read = 0;
    > +       /* pages are removed, resume tracing and then free the pages */
    > +       atomic_dec(&cpu_buffer->record_disabled);
    >        spin_unlock_irq(&cpu_buffer->reader_lock);
    > +
    > +       RB_WARN_ON(cpu_buffer, list_empty(cpu_buffer->pages));
    > +
    > +       /* last buffer page to remove */
    > +       last_page = list_entry(rb_list_head(to_remove), struct buffer_page,
    > +                               list);
    > +       tmp_iter_page = first_page;
    > +       do {
    > +               to_remove_page = tmp_iter_page;
    > +               rb_inc_page(cpu_buffer, &tmp_iter_page);
    > +               /* update the counters */
    > +               page_entries = rb_page_entries(to_remove_page);
    > +               if (page_entries) {
    > +                       /*
    > +                        * If something was added to this page, it was full
    > +                        * since it is not the tail page. So we deduct the
    > +                        * bytes consumed in ring buffer from here.
    > +                        * No need to update overruns, since this page is
    > +                        * deleted from ring buffer and its entries are
    > +                        * already accounted for.
    > +                        */
    > +                       local_sub(BUF_PAGE_SIZE, &cpu_buffer->entries_bytes);
    > +               }
    > +               /*
    > +                * We have already removed references to this list item, just
    > +                * free up the buffer_page and its page
    > +                */
    > +               nr_removed--;
    > +               free_buffer_page(to_remove_page);
    > +       } while (to_remove_page != last_page);
    > +
    > +       RB_WARN_ON(cpu_buffer, nr_removed);
    >  }
    >
    >  static void
    > @@ -1303,6 +1384,12 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer,
    >        struct list_head *p;
    >        unsigned i;
    >
    > +       /* stop the writers while inserting pages */
    > +       atomic_inc(&cpu_buffer->record_disabled);
    > +
    > +       /* Make sure all writers are done with this buffer. */
    > +       synchronize_sched();
    > +
    >        spin_lock_irq(&cpu_buffer->reader_lock);
    >        rb_head_page_deactivate(cpu_buffer);
    >
    > @@ -1319,17 +1406,21 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer,
    >
    >  out:
    >        spin_unlock_irq(&cpu_buffer->reader_lock);
    > +       atomic_dec(&cpu_buffer->record_disabled);
    >  }
    >
    > -static void update_pages_handler(struct ring_buffer_per_cpu *cpu_buffer)
    > +static void update_pages_handler(struct work_struct *work)
    >  {
    > +       struct ring_buffer_per_cpu *cpu_buffer = container_of(work,
    > +                       struct ring_buffer_per_cpu, update_pages_work);
    > +
    >        if (cpu_buffer->nr_pages_to_update > 0)
    >                rb_insert_pages(cpu_buffer, &cpu_buffer->new_pages,
    >                                cpu_buffer->nr_pages_to_update);
    >        else
    >                rb_remove_pages(cpu_buffer, -cpu_buffer->nr_pages_to_update);
    > -       /* reset this value */
    > -       cpu_buffer->nr_pages_to_update = 0;
    > +
    > +       complete(&cpu_buffer->update_completion);
    >  }
    >
    >  /**
    > @@ -1339,14 +1430,14 @@ static void update_pages_handler(struct ring_buffer_per_cpu *cpu_buffer)
    >  *
    >  * Minimum size is 2 * BUF_PAGE_SIZE.
    >  *
    > - * Returns -1 on failure.
    > + * Returns 0 on success and < 0 on failure.
    >  */
    >  int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
    >                        int cpu_id)
    >  {
    >        struct ring_buffer_per_cpu *cpu_buffer;
    >        unsigned nr_pages;
    > -       int cpu;
    > +       int cpu, err = 0;
    >
    >        /*
    >         * Always succeed at resizing a non-existent buffer:
    > @@ -1361,21 +1452,28 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
    >        if (size < BUF_PAGE_SIZE * 2)
    >                size = BUF_PAGE_SIZE * 2;
    >
    > -       atomic_inc(&buffer->record_disabled);
    > -
    > -       /* Make sure all writers are done with this buffer. */
    > -       synchronize_sched();
    > +       nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
    >
    > +       /*
    > +        * Don't succeed if recording is disabled globally, as a reader might
    > +        * be manipulating the ring buffer and is expecting a sane state while
    > +        * this is true.
    > +        */
    > +       if (atomic_read(&buffer->record_disabled))
    > +               return -EBUSY;
    > +       /* prevent another thread from changing buffer sizes */
    >        mutex_lock(&buffer->mutex);
    > -       get_online_cpus();
    > -
    > -       nr_pages = DIV_ROUND_UP(size, BUF_PAGE_SIZE);
    >
    >        if (cpu_id == RING_BUFFER_ALL_CPUS) {
    >                /* calculate the pages to update */
    >                for_each_buffer_cpu(buffer, cpu) {
    >                        cpu_buffer = buffer->buffers[cpu];
    >
    > +                       if (atomic_read(&cpu_buffer->record_disabled)) {
    > +                               err = -EBUSY;
    > +                               goto out_err;
    > +                       }
    > +
    >                        cpu_buffer->nr_pages_to_update = nr_pages -
    >                                                        cpu_buffer->nr_pages;
    >
    > @@ -1391,21 +1489,38 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
    >                         */
    >                        INIT_LIST_HEAD(&cpu_buffer->new_pages);
    >                        if (__rb_allocate_pages(cpu_buffer->nr_pages_to_update,
    > -                                               &cpu_buffer->new_pages, cpu))
    > +                                               &cpu_buffer->new_pages, cpu)) {
    >                                /* not enough memory for new pages */
    > -                               goto no_mem;
    > +                               err = -ENOMEM;
    > +                               goto out_err;
    > +                       }
    > +               }
    > +
    > +               /* fire off all the required work handlers */
    > +               for_each_buffer_cpu(buffer, cpu) {
    > +                       cpu_buffer = buffer->buffers[cpu];
    > +                       if (!cpu_buffer->nr_pages_to_update)
    > +                               continue;
    > +                       schedule_work_on(cpu, &cpu_buffer->update_pages_work);
    >                }
    >
    >                /* wait for all the updates to complete */
    >                for_each_buffer_cpu(buffer, cpu) {
    >                        cpu_buffer = buffer->buffers[cpu];
    > -                       if (cpu_buffer->nr_pages_to_update) {
    > -                               update_pages_handler(cpu_buffer);
    > -                               cpu_buffer->nr_pages = nr_pages;
    > -                       }
    > +                       if (!cpu_buffer->nr_pages_to_update)
    > +                               continue;
    > +                       wait_for_completion(&cpu_buffer->update_completion);
    > +                       cpu_buffer->nr_pages = nr_pages;
    > +                       /* reset this value */
    > +                       cpu_buffer->nr_pages_to_update = 0;
    >                }
    >        } else {
    >                cpu_buffer = buffer->buffers[cpu_id];
    > +               if (atomic_read(&cpu_buffer->record_disabled)) {
    > +                       err = -EBUSY;
    > +                       goto out_err;
    > +               }
    > +
    >                if (nr_pages == cpu_buffer->nr_pages)
    >                        goto out;
    >
    > @@ -1415,40 +1530,42 @@ int ring_buffer_resize(struct ring_buffer *buffer, unsigned long size,
    >                INIT_LIST_HEAD(&cpu_buffer->new_pages);
    >                if (cpu_buffer->nr_pages_to_update > 0 &&
    >                        __rb_allocate_pages(cpu_buffer->nr_pages_to_update,
    > -                                               &cpu_buffer->new_pages, cpu_id))
    > -                       goto no_mem;
    > +                                       &cpu_buffer->new_pages, cpu_id)) {
    > +                       err = -ENOMEM;
    > +                       goto out_err;
    > +               }
    >
    > -               update_pages_handler(cpu_buffer);
    > +               schedule_work_on(cpu_id, &cpu_buffer->update_pages_work);
    > +               wait_for_completion(&cpu_buffer->update_completion);
    >
    >                cpu_buffer->nr_pages = nr_pages;
    > +               /* reset this value */
    > +               cpu_buffer->nr_pages_to_update = 0;
    >        }
    >
    >  out:
    > -       put_online_cpus();
    >        mutex_unlock(&buffer->mutex);
    > -
    > -       atomic_dec(&buffer->record_disabled);
    > -
    >        return size;
    >
    > - no_mem:
    > + out_err:
    >        for_each_buffer_cpu(buffer, cpu) {
    >                struct buffer_page *bpage, *tmp;
    > +
    >                cpu_buffer = buffer->buffers[cpu];
    >                /* reset this number regardless */
    >                cpu_buffer->nr_pages_to_update = 0;
    > +
    >                if (list_empty(&cpu_buffer->new_pages))
    >                        continue;
    > +
    >                list_for_each_entry_safe(bpage, tmp, &cpu_buffer->new_pages,
    >                                        list) {
    >                        list_del_init(&bpage->list);
    >                        free_buffer_page(bpage);
    >                }
    >        }
    > -       put_online_cpus();
    >        mutex_unlock(&buffer->mutex);
    > -       atomic_dec(&buffer->record_disabled);
    > -       return -ENOMEM;
    > +       return err;
    >  }
    >  EXPORT_SYMBOL_GPL(ring_buffer_resize);
    >
    > @@ -1487,21 +1604,11 @@ rb_iter_head_event(struct ring_buffer_iter *iter)
    >        return __rb_page_index(iter->head_page, iter->head);
    >  }
    >
    > -static inline unsigned long rb_page_write(struct buffer_page *bpage)
    > -{
    > -       return local_read(&bpage->write) & RB_WRITE_MASK;
    > -}
    > -
    >  static inline unsigned rb_page_commit(struct buffer_page *bpage)
    >  {
    >        return local_read(&bpage->page->commit);
    >  }
    >
    > -static inline unsigned long rb_page_entries(struct buffer_page *bpage)
    > -{
    > -       return local_read(&bpage->entries) & RB_WRITE_MASK;
    > -}
    > -
    >  /* Size is determined by what has been committed */
    >  static inline unsigned rb_page_size(struct buffer_page *bpage)
    >  {
    > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
    > index bb3c867..736518f 100644
    > --- a/kernel/trace/trace.c
    > +++ b/kernel/trace/trace.c
    > @@ -2936,20 +2936,10 @@ static int __tracing_resize_ring_buffer(unsigned long size, int cpu)
    >
    >  static ssize_t tracing_resize_ring_buffer(unsigned long size, int cpu_id)
    >  {
    > -       int cpu, ret = size;
    > +       int ret = size;
    >
    >        mutex_lock(&trace_types_lock);
    >
    > -       tracing_stop();
    > -
    > -       /* disable all cpu buffers */
    > -       for_each_tracing_cpu(cpu) {
    > -               if (global_trace.data[cpu])
    > -                       atomic_inc(&global_trace.data[cpu]->disabled);
    > -               if (max_tr.data[cpu])
    > -                       atomic_inc(&max_tr.data[cpu]->disabled);
    > -       }
    > -
    >        if (cpu_id != RING_BUFFER_ALL_CPUS) {
    >                /* make sure, this cpu is enabled in the mask */
    >                if (!cpumask_test_cpu(cpu_id, tracing_buffer_mask)) {
    > @@ -2963,14 +2953,6 @@ static ssize_t tracing_resize_ring_buffer(unsigned long size, int cpu_id)
    >                ret = -ENOMEM;
    >
    >  out:
    > -       for_each_tracing_cpu(cpu) {
    > -               if (global_trace.data[cpu])
    > -                       atomic_dec(&global_trace.data[cpu]->disabled);
    > -               if (max_tr.data[cpu])
    > -                       atomic_dec(&max_tr.data[cpu]->disabled);
    > -       }
    > -
    > -       tracing_start();
    >        mutex_unlock(&trace_types_lock);
    >
    >        return ret;
    > --
    > 1.7.3.1
    >
    >
    \
     
     \ /
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