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    SubjectRe: [PATCH v2 1/4] perf: Allow using AUX data in perf samples
    On Tue, Oct 22, 2019 at 12:58:09PM +0300, Alexander Shishkin wrote:
    > AUX data can be used to annotate perf events such as performance counters
    > or tracepoints/breakpoints by including it in sample records when
    > PERF_SAMPLE_AUX flag is set. Such samples would be instrumental in debugging
    > and profiling by providing, for example, a history of instruction flow
    > leading up to the event's overflow.
    >
    > The implementation makes use of grouping an AUX event with all the events
    > that wish to take samples of the AUX data, such that the former is the
    > group leader. The samplees should also specify the desired size of the AUX
    > sample via attr.aux_sample_size.
    >
    > AUX capable PMUs need to explicitly add support for sampling, because it
    > relies on a new callback to take a snapshot of the buffer without touching
    > the event states.

    Mathieu (and other ARM-CS folks) is this something that can work for
    you? It would be really bad to discover this interface can't work for
    you after it's been merged.

    > Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
    > ---
    > include/linux/perf_event.h | 20 ++++
    > include/uapi/linux/perf_event.h | 7 +-
    > kernel/events/core.c | 159 +++++++++++++++++++++++++++++++-
    > kernel/events/internal.h | 1 +
    > kernel/events/ring_buffer.c | 36 ++++++++
    > 5 files changed, 220 insertions(+), 3 deletions(-)
    >
    > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    > index 587ae4d002f5..24c5093f1229 100644
    > --- a/include/linux/perf_event.h
    > +++ b/include/linux/perf_event.h
    > @@ -249,6 +249,8 @@ struct perf_event;
    > #define PERF_PMU_CAP_NO_EXCLUDE 0x80
    > #define PERF_PMU_CAP_AUX_OUTPUT 0x100
    >
    > +struct perf_output_handle;
    > +
    > /**
    > * struct pmu - generic performance monitoring unit
    > */
    > @@ -423,6 +425,19 @@ struct pmu {
    > */
    > void (*free_aux) (void *aux); /* optional */
    >
    > + /*
    > + * Take a snapshot of the AUX buffer without touching the event
    > + * state, so that preempting ->start()/->stop() callbacks does
    > + * not interfere with their logic. Called in PMI context.
    > + *
    > + * Returns the size of AUX data copied to the output handle.
    > + *
    > + * Optional.
    > + */
    > + long (*snapshot_aux) (struct perf_event *event,
    > + struct perf_output_handle *handle,
    > + unsigned long size);
    > +
    > /*
    > * Validate address range filters: make sure the HW supports the
    > * requested configuration and number of filters; return 0 if the
    > @@ -964,6 +979,7 @@ struct perf_sample_data {
    > u32 reserved;
    > } cpu_entry;
    > struct perf_callchain_entry *callchain;
    > + u64 aux_size;
    >
    > /*
    > * regs_user may point to task_pt_regs or to regs_user_copy, depending
    > @@ -996,6 +1012,7 @@ static inline void perf_sample_data_init(struct perf_sample_data *data,
    > data->weight = 0;
    > data->data_src.val = PERF_MEM_NA;
    > data->txn = 0;
    > + data->aux_size = 0;
    > }
    >
    > extern void perf_output_sample(struct perf_output_handle *handle,
    > @@ -1353,6 +1370,9 @@ extern unsigned int perf_output_copy(struct perf_output_handle *handle,
    > const void *buf, unsigned int len);
    > extern unsigned int perf_output_skip(struct perf_output_handle *handle,
    > unsigned int len);
    > +extern long perf_output_copy_aux(struct perf_output_handle *aux_handle,
    > + struct perf_output_handle *handle,
    > + unsigned long from, unsigned long to);
    > extern int perf_swevent_get_recursion_context(void);
    > extern void perf_swevent_put_recursion_context(int rctx);
    > extern u64 perf_swevent_set_period(struct perf_event *event);
    > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
    > index bb7b271397a6..84dbd1499a24 100644
    > --- a/include/uapi/linux/perf_event.h
    > +++ b/include/uapi/linux/perf_event.h
    > @@ -141,8 +141,9 @@ enum perf_event_sample_format {
    > PERF_SAMPLE_TRANSACTION = 1U << 17,
    > PERF_SAMPLE_REGS_INTR = 1U << 18,
    > PERF_SAMPLE_PHYS_ADDR = 1U << 19,
    > + PERF_SAMPLE_AUX = 1U << 20,
    >
    > - PERF_SAMPLE_MAX = 1U << 20, /* non-ABI */
    > + PERF_SAMPLE_MAX = 1U << 21, /* non-ABI */
    >
    > __PERF_SAMPLE_CALLCHAIN_EARLY = 1ULL << 63, /* non-ABI; internal use */
    > };
    > @@ -424,7 +425,7 @@ struct perf_event_attr {
    > */
    > __u32 aux_watermark;
    > __u16 sample_max_stack;
    > - __u16 __reserved_2; /* align to __u64 */
    > + __u16 aux_sample_size;
    > };
    >
    > /*
    > @@ -864,6 +865,8 @@ enum perf_event_type {
    > * { u64 abi; # enum perf_sample_regs_abi
    > * u64 regs[weight(mask)]; } && PERF_SAMPLE_REGS_INTR
    > * { u64 phys_addr;} && PERF_SAMPLE_PHYS_ADDR
    > + * { u64 size;
    > + * char data[size]; } && PERF_SAMPLE_AUX
    > * };
    > */
    > PERF_RECORD_SAMPLE = 9,
    > diff --git a/kernel/events/core.c b/kernel/events/core.c
    > index 77793ef0d8bc..5e2c7a715434 100644
    > --- a/kernel/events/core.c
    > +++ b/kernel/events/core.c
    > @@ -1953,7 +1953,10 @@ static int perf_get_aux_event(struct perf_event *event,
    > if (!group_leader)
    > return 0;
    >
    > - if (!perf_aux_output_match(event, group_leader))
    > + if (event->attr.aux_output && !perf_aux_output_match(event, group_leader))
    > + return 0;
    > +
    > + if (event->attr.aux_sample_size && !group_leader->pmu->snapshot_aux)
    > return 0;
    >
    > if (!atomic_long_inc_not_zero(&group_leader->refcount))
    > @@ -6192,6 +6195,121 @@ perf_output_sample_ustack(struct perf_output_handle *handle, u64 dump_size,
    > }
    > }
    >
    > +static unsigned long perf_aux_sample_size(struct perf_event *event,
    > + struct perf_sample_data *data,
    > + size_t size)
    > +{
    > + struct perf_event *sampler = event->aux_event;
    > + struct ring_buffer *rb;
    > +
    > + data->aux_size = 0;
    > +
    > + if (!sampler)
    > + goto out;
    > +
    > + if (WARN_ON_ONCE(READ_ONCE(sampler->state) != PERF_EVENT_STATE_ACTIVE))
    > + goto out;
    > +
    > + if (WARN_ON_ONCE(READ_ONCE(sampler->oncpu) != smp_processor_id()))
    > + goto out;
    > +
    > + rb = ring_buffer_get(sampler->parent ? sampler->parent : sampler);
    > + if (!rb)
    > + goto out;
    > +
    > + /*
    > + * If this is an NMI hit inside sampling code, don't take
    > + * the sample. See also perf_aux_sample_output().
    > + */
    > + if (READ_ONCE(rb->aux_in_sampling)) {
    > + data->aux_size = 0;
    > + } else {
    > + size = min_t(size_t, size, perf_aux_size(rb));
    > + data->aux_size = ALIGN(size, sizeof(u64));
    > + }
    > + ring_buffer_put(rb);
    > +
    > +out:
    > + return data->aux_size;
    > +}
    > +
    > +long perf_pmu_aux_sample_output(struct perf_event *event,
    > + struct perf_output_handle *handle,
    > + unsigned long size)
    > +{
    > + unsigned long flags;
    > + long ret;
    > +
    > + /*
    > + * Normal ->start()/->stop() callbacks run in IRQ mode in scheduler
    > + * paths. If we start calling them in NMI context, they may race with
    > + * the IRQ ones, that is, for example, re-starting an event that's just
    > + * been stopped, which is why we're using a separate callback that
    > + * doesn't change the event state.
    > + *
    > + * IRQs need to be disabled to prevent IPIs from racing with us.
    > + */
    > + local_irq_save(flags);
    > +
    > + ret = event->pmu->snapshot_aux(event, handle, size);
    > +
    > + local_irq_restore(flags);
    > +
    > + return ret;
    > +}
    > +
    > +static void perf_aux_sample_output(struct perf_event *event,
    > + struct perf_output_handle *handle,
    > + struct perf_sample_data *data)
    > +{
    > + struct perf_event *sampler = event->aux_event;
    > + unsigned long pad;
    > + struct ring_buffer *rb;
    > + long size;
    > +
    > + if (WARN_ON_ONCE(!sampler || !data->aux_size))
    > + return;
    > +
    > + rb = ring_buffer_get(sampler->parent ? sampler->parent : sampler);
    > + if (!rb)
    > + return;
    > +
    > + /*
    > + * Guard against NMI hits inside the critical section;
    > + * see also perf_aux_sample_size().
    > + */
    > + WRITE_ONCE(rb->aux_in_sampling, 1);
    > +
    > + size = perf_pmu_aux_sample_output(sampler, handle, data->aux_size);
    > +
    > + /*
    > + * An error here means that perf_output_copy() failed (returned a
    > + * non-zero surplus that it didn't copy), which in its current
    > + * enlightened implementation is not possible. If that changes, we'd
    > + * like to know.
    > + */
    > + if (WARN_ON_ONCE(size < 0))
    > + goto out_clear;
    > +
    > + /*
    > + * The pad comes from ALIGN()ing data->aux_size up to u64 in
    > + * perf_aux_sample_size(), so should not be more than that.
    > + */
    > + pad = data->aux_size - size;
    > + if (WARN_ON_ONCE(pad >= sizeof(u64)))
    > + pad = 8;
    > +
    > + if (pad) {
    > + u64 zero = 0;
    > + perf_output_copy(handle, &zero, pad);
    > + }
    > +
    > +out_clear:
    > + WRITE_ONCE(rb->aux_in_sampling, 0);
    > +
    > + ring_buffer_put(rb);
    > +}
    > +
    > static void __perf_event_header__init_id(struct perf_event_header *header,
    > struct perf_sample_data *data,
    > struct perf_event *event)
    > @@ -6511,6 +6629,13 @@ void perf_output_sample(struct perf_output_handle *handle,
    > if (sample_type & PERF_SAMPLE_PHYS_ADDR)
    > perf_output_put(handle, data->phys_addr);
    >
    > + if (sample_type & PERF_SAMPLE_AUX) {
    > + perf_output_put(handle, data->aux_size);
    > +
    > + if (data->aux_size)
    > + perf_aux_sample_output(event, handle, data);
    > + }
    > +
    > if (!event->attr.watermark) {
    > int wakeup_events = event->attr.wakeup_events;
    >
    > @@ -6699,6 +6824,35 @@ void perf_prepare_sample(struct perf_event_header *header,
    >
    > if (sample_type & PERF_SAMPLE_PHYS_ADDR)
    > data->phys_addr = perf_virt_to_phys(data->addr);
    > +
    > + if (sample_type & PERF_SAMPLE_AUX) {
    > + u64 size;
    > +
    > + header->size += sizeof(u64); /* size */
    > +
    > + /*
    > + * Given the 16bit nature of header::size, an AUX sample can
    > + * easily overflow it, what with all the preceding sample bits.
    > + * Make sure this doesn't happen by using up to U16_MAX bytes
    > + * per sample in total (rounded down to 8 byte boundary).
    > + */
    > + size = min_t(size_t, U16_MAX - header->size,
    > + event->attr.aux_sample_size);
    > + size = rounddown(size, 8);
    > + size = perf_aux_sample_size(event, data, size);
    > +
    > + WARN_ON_ONCE(size + header->size > U16_MAX);
    > + header->size += size;
    > + }
    > + /*
    > + * If you're adding more sample types here, you likely need to do
    > + * something about the overflowing header::size, like repurpose the
    > + * lowest 3 bits of size, which should be always zero at the moment.
    > + * This raises a more important question, do we really need 512k sized
    > + * samples and why, so good argumentation is in order for whatever you
    > + * do here next.
    > + */
    > + WARN_ON_ONCE(header->size & 7);
    > }
    >
    > static __always_inline int
    > @@ -11213,6 +11367,9 @@ SYSCALL_DEFINE5(perf_event_open,
    > if (event->attr.aux_output && !perf_get_aux_event(event, group_leader))
    > goto err_locked;
    >
    > + if (event->attr.aux_sample_size && !perf_get_aux_event(event, group_leader))
    > + goto err_locked;
    > +
    > /*
    > * Must be under the same ctx::mutex as perf_install_in_context(),
    > * because we need to serialize with concurrent event creation.
    > diff --git a/kernel/events/internal.h b/kernel/events/internal.h
    > index 3aef4191798c..747d67f130cb 100644
    > --- a/kernel/events/internal.h
    > +++ b/kernel/events/internal.h
    > @@ -50,6 +50,7 @@ struct ring_buffer {
    > unsigned long aux_mmap_locked;
    > void (*free_aux)(void *);
    > refcount_t aux_refcount;
    > + int aux_in_sampling;
    > void **aux_pages;
    > void *aux_priv;
    >
    > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
    > index 246c83ac5643..7ffd5c763f93 100644
    > --- a/kernel/events/ring_buffer.c
    > +++ b/kernel/events/ring_buffer.c
    > @@ -562,6 +562,42 @@ void *perf_get_aux(struct perf_output_handle *handle)
    > }
    > EXPORT_SYMBOL_GPL(perf_get_aux);
    >
    > +/*
    > + * Copy out AUX data from an AUX handle.
    > + */
    > +long perf_output_copy_aux(struct perf_output_handle *aux_handle,
    > + struct perf_output_handle *handle,
    > + unsigned long from, unsigned long to)
    > +{
    > + unsigned long tocopy, remainder, len = 0;
    > + struct ring_buffer *rb = aux_handle->rb;
    > + void *addr;
    > +
    > + from &= (rb->aux_nr_pages << PAGE_SHIFT) - 1;
    > + to &= (rb->aux_nr_pages << PAGE_SHIFT) - 1;
    > +
    > + do {
    > + tocopy = PAGE_SIZE - offset_in_page(from);
    > + if (to > from)
    > + tocopy = min(tocopy, to - from);
    > + if (!tocopy)
    > + break;
    > +
    > + addr = rb->aux_pages[from >> PAGE_SHIFT];
    > + addr += offset_in_page(from);
    > +
    > + remainder = perf_output_copy(handle, addr, tocopy);
    > + if (remainder)
    > + return -EFAULT;
    > +
    > + len += tocopy;
    > + from += tocopy;
    > + from &= (rb->aux_nr_pages << PAGE_SHIFT) - 1;
    > + } while (to != from);
    > +
    > + return len;
    > +}
    > +
    > #define PERF_AUX_GFP (GFP_KERNEL | __GFP_ZERO | __GFP_NOWARN | __GFP_NORETRY)
    >
    > static struct page *rb_alloc_aux_page(int node, int order)
    > --
    > 2.23.0
    >

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