Messages in this thread Patch in this message | | | Date | Tue, 27 Oct 2015 14:43:45 +0800 | From | xiakaixu <> | Subject | Re: [PATCH V5 1/1] bpf: control events stored in PERF_EVENT_ARRAY maps trace data output when perf sampling |
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于 2015/10/23 23:12, Peter Zijlstra 写道: > On Fri, Oct 23, 2015 at 02:52:11PM +0200, Peter Zijlstra wrote: >> On Thu, Oct 22, 2015 at 06:28:22PM +0800, Wangnan (F) wrote: >>> information to analysis when glitch happen. Another way we are trying to >>> implement >>> now is to dynamically turn events on and off, or at least enable/disable >>> sampling dynamically because the overhead of copying those samples >>> is a big part of perf's total overhead. After that we can trace as many >>> event as possible, but only fetch data from them when we detect a glitch. >> >> So why don't you 'fix' the flight recorder mode and just leave the data >> in memory and not bother copying it out until a glitch happens? >> >> Something like this: >> >> lkml.kernel.org/r/20130708121557.GA17211@twins.programming.kicks-ass.net >> >> it appears we never quite finished that. > > Updated to current sources, compile tested only. > > It obviously needs testing and performance numbers.. and some > userspace. > > --- > Subject: perf: Update event buffer tail when overwriting old events > From: Peter Zijlstra <peterz@infradead.org> > >> From: "Yan, Zheng" <zheng.z.yan@intel.com> >> >> If perf event buffer is in overwrite mode, the kernel only updates >> the data head when it overwrites old samples. The program that owns >> the buffer need periodically check the buffer and update a variable >> that tracks the date tail. If the program fails to do this in time, >> the data tail can be overwritted by new samples. The program has to >> rewind the buffer because it does not know where is the first vaild >> sample. >> >> This patch makes the kernel update the date tail when it overwrites >> old events. So the program that owns the event buffer can always >> read the latest samples. This is convenient for programs that use >> perf to do branch tracing. One use case is GDB branch tracing: >> (http://sourceware.org/ml/gdb-patches/2012-06/msg00172.html) >> It uses perf interface to read BTS, but only cares the branches >> before the ptrace event. > > Original-patch-by: "Yan, Zheng" <zheng.z.yan@intel.com> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > arch/x86/kernel/cpu/perf_event_intel_ds.c | 2 > include/linux/perf_event.h | 6 -- > kernel/events/core.c | 56 +++++++++++++++++---- > kernel/events/internal.h | 2 > kernel/events/ring_buffer.c | 77 +++++++++++++++++++++--------- > 5 files changed, 107 insertions(+), 36 deletions(-) > > --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c > +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c > @@ -1140,7 +1140,7 @@ static void __intel_pmu_pebs_event(struc > > while (count > 1) { > setup_pebs_sample_data(event, iregs, at, &data, ®s); > - perf_event_output(event, &data, ®s); > + event->overflow_handler(event, &data, ®s); > at += x86_pmu.pebs_record_size; > at = get_next_pebs_record_by_bit(at, top, bit); > count--; > --- a/include/linux/perf_event.h > +++ b/include/linux/perf_event.h > @@ -828,10 +828,6 @@ extern int perf_event_overflow(struct pe > struct perf_sample_data *data, > struct pt_regs *regs); > > -extern void perf_event_output(struct perf_event *event, > - struct perf_sample_data *data, > - struct pt_regs *regs); > - > extern void > perf_event_header__init_id(struct perf_event_header *header, > struct perf_sample_data *data, > @@ -1032,6 +1028,8 @@ static inline bool has_aux(struct perf_e > > extern int perf_output_begin(struct perf_output_handle *handle, > struct perf_event *event, unsigned int size); > +extern int perf_output_begin_overwrite(struct perf_output_handle *handle, > + struct perf_event *event, unsigned int size); > extern void perf_output_end(struct perf_output_handle *handle); > extern unsigned int perf_output_copy(struct perf_output_handle *handle, > const void *buf, unsigned int len); > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -4515,6 +4515,8 @@ static int perf_mmap_fault(struct vm_are > return ret; > } > > +static void perf_event_set_overflow(struct perf_event *event, struct ring_buffer *rb); > + > static void ring_buffer_attach(struct perf_event *event, > struct ring_buffer *rb) > { > @@ -4546,6 +4548,8 @@ static void ring_buffer_attach(struct pe > spin_lock_irqsave(&rb->event_lock, flags); > list_add_rcu(&event->rb_entry, &rb->event_list); > spin_unlock_irqrestore(&rb->event_lock, flags); > + > + perf_event_set_overflow(event, rb); > } > > rcu_assign_pointer(event->rb, rb); > @@ -5579,9 +5583,12 @@ void perf_prepare_sample(struct perf_eve > } > } > > -void perf_event_output(struct perf_event *event, > - struct perf_sample_data *data, > - struct pt_regs *regs) > +static __always_inline void > +__perf_event_output(struct perf_event *event, > + struct perf_sample_data *data, > + struct pt_regs *regs, > + int (*output_begin)(struct perf_output_handle *, > + struct perf_event *, unsigned int)) > { > struct perf_output_handle handle; > struct perf_event_header header; > @@ -5591,7 +5598,7 @@ void perf_event_output(struct perf_event > > perf_prepare_sample(&header, data, event, regs); > > - if (perf_output_begin(&handle, event, header.size)) > + if (output_begin(&handle, event, header.size)) > goto exit; > > perf_output_sample(&handle, &header, data, event); > @@ -5602,6 +5609,33 @@ void perf_event_output(struct perf_event > rcu_read_unlock(); > } > > +static void perf_event_output(struct perf_event *event, > + struct perf_sample_data *data, > + struct pt_regs *regs) > +{ > + __perf_event_output(event, data, regs, perf_output_begin); > +} > + > +static void perf_event_output_overwrite(struct perf_event *event, > + struct perf_sample_data *data, > + struct pt_regs *regs) > +{ > + __perf_event_output(event, data, regs, perf_output_begin_overwrite); > +} > + > +static void > +perf_event_set_overflow(struct perf_event *event, struct ring_buffer *rb) > +{ > + if (event->overflow_handler != perf_event_output && > + event->overflow_handler != perf_event_output_overwrite) > + return; > + > + if (rb->overwrite) > + event->overflow_handler = perf_event_output_overwrite; > + else > + event->overflow_handler = perf_event_output; > +} > + > /* > * read event_id > */ > @@ -6426,10 +6460,7 @@ static int __perf_event_overflow(struct > irq_work_queue(&event->pending); > } > > - if (event->overflow_handler) > - event->overflow_handler(event, data, regs); > - else > - perf_event_output(event, data, regs); > + event->overflow_handler(event, data, regs); > > if (*perf_event_fasync(event) && event->pending_kill) { > event->pending_wakeup = 1; > @@ -7904,8 +7935,13 @@ perf_event_alloc(struct perf_event_attr > context = parent_event->overflow_handler_context; > } > > - event->overflow_handler = overflow_handler; > - event->overflow_handler_context = context; > + if (overflow_handler) { > + event->overflow_handler = overflow_handler; > + event->overflow_handler_context = context; > + } else { > + event->overflow_handler = perf_event_output; > + event->overflow_handler_context = NULL; > + } > > perf_event__state_init(event); > > --- a/kernel/events/internal.h > +++ b/kernel/events/internal.h > @@ -21,6 +21,8 @@ struct ring_buffer { > > atomic_t poll; /* POLL_ for wakeups */ > > + local_t tail; /* read position */ > + local_t next_tail; /* next read position */ > local_t head; /* write position */ > local_t nest; /* nested writers */ > local_t events; /* event limit */ > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -102,11 +102,11 @@ static void perf_output_put_handle(struc > preempt_enable(); > } > > -int perf_output_begin(struct perf_output_handle *handle, > - struct perf_event *event, unsigned int size) > +static __always_inline int __perf_output_begin(struct perf_output_handle *handle, > + struct perf_event *event, unsigned int size, bool overwrite) > { > struct ring_buffer *rb; > - unsigned long tail, offset, head; > + unsigned long tail, offset, head, max_size; > int have_lost, page_shift; > struct { > struct perf_event_header header; > @@ -125,7 +125,8 @@ int perf_output_begin(struct perf_output > if (unlikely(!rb)) > goto out; > > - if (unlikely(!rb->nr_pages)) > + max_size = perf_data_size(rb); > + if (unlikely(size > max_size)) > goto out; > > handle->rb = rb; > @@ -140,27 +141,49 @@ int perf_output_begin(struct perf_output > > perf_output_get_handle(handle); > > - do { > - tail = READ_ONCE_CTRL(rb->user_page->data_tail); > - offset = head = local_read(&rb->head); > - if (!rb->overwrite && > - unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size)) > - goto fail; > + if (overwrite) { > + do { > + tail = local_read(&rb->tail); > + offset = local_read(&rb->head); > + head = offset + size; > + if (unlikely(CIRC_SPACE(head, tail, max_size) < size)) { Should be 'if (unlikely(CIRC_SPACE(offset, tail, max_size) < size))'?
> + tail = local_read(&rb->next_tail); > + local_set(&rb->tail, tail); > + rb->user_page->data_tail = tail; > + } > + } while (local_cmpxchg(&rb->head, offset, head) != offset); > > /* > - * The above forms a control dependency barrier separating the > - * @tail load above from the data stores below. Since the @tail > - * load is required to compute the branch to fail below. > - * > - * A, matches D; the full memory barrier userspace SHOULD issue > - * after reading the data and before storing the new tail > - * position. > - * > - * See perf_output_put_handle(). > + * Save the start of next event when half of the buffer > + * has been filled. Later when the event buffer overflows, > + * update the tail pointer to point to it. > */ > + if (tail == local_read(&rb->next_tail) && > + CIRC_CNT(head, tail, max_size) >= (max_size / 2)) > + local_cmpxchg(&rb->next_tail, tail, head); > + } else { > + do { > + tail = READ_ONCE_CTRL(rb->user_page->data_tail); > + offset = head = local_read(&rb->head); > + if (!rb->overwrite && > + unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size)) > + goto fail; > + > + /* > + * The above forms a control dependency barrier separating the > + * @tail load above from the data stores below. Since the @tail > + * load is required to compute the branch to fail below. > + * > + * A, matches D; the full memory barrier userspace SHOULD issue > + * after reading the data and before storing the new tail > + * position. > + * > + * See perf_output_put_handle(). > + */ > > - head += size; > - } while (local_cmpxchg(&rb->head, offset, head) != offset); > + head += size; > + } while (local_cmpxchg(&rb->head, offset, head) != offset); > + } > > /* > * We rely on the implied barrier() by local_cmpxchg() to ensure > @@ -203,6 +226,18 @@ int perf_output_begin(struct perf_output > return -ENOSPC; > } > > +int perf_output_begin(struct perf_output_handle *handle, > + struct perf_event *event, unsigned int size) > +{ > + return __perf_output_begin(handle, event, size, false); > +} > + > +int perf_output_begin_overwrite(struct perf_output_handle *handle, > + struct perf_event *event, unsigned int size) > +{ > + return __perf_output_begin(handle, event, size, true); > +} > + > unsigned int perf_output_copy(struct perf_output_handle *handle, > const void *buf, unsigned int len) > { > > . >
-- Regards Kaixu Xia
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