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Message-ID: <20241006103947.GA30834@debian-dev>
Date: Sun, 6 Oct 2024 18:39:47 +0800
From: Leo Yan <leo.yan@...ux.dev>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Thomas Richter <tmricht@...ux.ibm.com>,
Hendrik Brueckner <brueckner@...ux.ibm.com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Mike Leach <mike.leach@...aro.org>,
James Clark <james.clark@....com>, coresight@...ts.linaro.org,
linux-arm-kernel@...ts.infradead.org,
Yicong Yang <yangyicong@...ilicon.com>,
Jonathan Cameron <jonathan.cameron@...wei.com>,
Will Deacon <will@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
Ian Rogers <irogers@...gle.com>, Andi Kleen <ak@...ux.intel.com>,
linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH V11 01/10] perf/core: Add aux_pause, aux_resume,
aux_start_paused
Hi Adrian,
Sorry I review this series in very late time and I might bring up
duplicate questions (though I hope not).
On Tue, Aug 06, 2024 at 06:55:05PM +0300, Adrian Hunter wrote:
> Hardware traces, such as instruction traces, can produce a vast amount of
> trace data, so being able to reduce tracing to more specific circumstances
> can be useful.
>
> The ability to pause or resume tracing when another event happens, can do
> that.
>
> Add ability for an event to "pause" or "resume" AUX area tracing.
>
> Add aux_pause bit to perf_event_attr to indicate that, if the event
> happens, the associated AUX area tracing should be paused. Ditto
> aux_resume. Do not allow aux_pause and aux_resume to be set together.
>
> Add aux_start_paused bit to perf_event_attr to indicate to an AUX area
> event that it should start in a "paused" state.
>
> Add aux_paused to struct hw_perf_event for AUX area events to keep track of
> the "paused" state. aux_paused is initialized to aux_start_paused.
>
> Add PERF_EF_PAUSE and PERF_EF_RESUME modes for ->stop() and ->start()
> callbacks. Call as needed, during __perf_event_output(). Add
> aux_in_pause_resume to struct perf_buffer to prevent races with the NMI
> handler. Pause/resume in NMI context will miss out if it coincides with
> another pause/resume.
>
> To use aux_pause or aux_resume, an event must be in a group with the AUX
> area event as the group leader.
>
> Example (requires Intel PT and tools patches also):
>
> $ perf record --kcore -e intel_pt/aux-action=start-paused/k,syscalls:sys_enter_newuname/aux-action=resume/,syscalls:sys_exit_newuname/aux-action=pause/ uname
> Linux
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.043 MB perf.data ]
> $ perf script --call-trace
> uname 30805 [000] 24001.058782799: name: 0x7ffc9c1865b0
> uname 30805 [000] 24001.058784424: psb offs: 0
> uname 30805 [000] 24001.058784424: cbr: 39 freq: 3904 MHz (139%)
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) debug_smp_processor_id
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) __x64_sys_newuname
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) down_read
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) __cond_resched
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) preempt_count_add
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) in_lock_functions
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) preempt_count_sub
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) up_read
> uname 30805 [000] 24001.058784629: ([kernel.kallsyms]) preempt_count_add
> uname 30805 [000] 24001.058784838: ([kernel.kallsyms]) in_lock_functions
> uname 30805 [000] 24001.058784838: ([kernel.kallsyms]) preempt_count_sub
> uname 30805 [000] 24001.058784838: ([kernel.kallsyms]) _copy_to_user
> uname 30805 [000] 24001.058784838: ([kernel.kallsyms]) syscall_exit_to_user_mode
> uname 30805 [000] 24001.058784838: ([kernel.kallsyms]) syscall_exit_work
> uname 30805 [000] 24001.058784838: ([kernel.kallsyms]) perf_syscall_exit
> uname 30805 [000] 24001.058784838: ([kernel.kallsyms]) debug_smp_processor_id
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) perf_trace_buf_alloc
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) perf_swevent_get_recursion_context
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) debug_smp_processor_id
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) debug_smp_processor_id
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) perf_tp_event
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) perf_trace_buf_update
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) tracing_gen_ctx_irq_test
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) perf_swevent_event
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) __perf_event_account_interrupt
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) __this_cpu_preempt_check
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) perf_event_output_forward
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) perf_event_aux_pause
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) ring_buffer_get
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) __rcu_read_lock
> uname 30805 [000] 24001.058785046: ([kernel.kallsyms]) __rcu_read_unlock
> uname 30805 [000] 24001.058785254: ([kernel.kallsyms]) pt_event_stop
> uname 30805 [000] 24001.058785254: ([kernel.kallsyms]) debug_smp_processor_id
> uname 30805 [000] 24001.058785254: ([kernel.kallsyms]) debug_smp_processor_id
> uname 30805 [000] 24001.058785254: ([kernel.kallsyms]) native_write_msr
> uname 30805 [000] 24001.058785463: ([kernel.kallsyms]) native_write_msr
> uname 30805 [000] 24001.058785639: 0x0
>
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> Acked-by: James Clark <james.clark@....com>
> ---
>
>
> Changes in V11:
> Make assignment to event->hw.aux_paused conditional on
> (pmu->capabilities & PERF_PMU_CAP_AUX_PAUSE).
>
> Changes in V10:
> Move aux_paused into a union within struct hw_perf_event.
> Additional comment wrt PERF_EF_PAUSE/PERF_EF_RESUME.
> Factor out has_aux_action() as an inline function.
> Use scoped_guard for irqsave.
> Move calls of perf_event_aux_pause() from __perf_event_output()
> to __perf_event_overflow().
>
> Changes in V9:
> Move aux_paused to struct hw_perf_event
>
> Changes in V6:
> Removed READ/WRITE_ONCE from __perf_event_aux_pause()
> Expanded comment about guarding against NMI
>
> Changes in V5:
> Added James' Ack
>
> Changes in V4:
> Rename aux_output_cfg -> aux_action
> Reorder aux_action bits from:
> aux_pause, aux_resume, aux_start_paused
> to:
> aux_start_paused, aux_pause, aux_resume
> Fix aux_action bits __u64 -> __u32
>
>
> include/linux/perf_event.h | 28 +++++++++++++
> include/uapi/linux/perf_event.h | 11 ++++-
> kernel/events/core.c | 72 +++++++++++++++++++++++++++++++--
> kernel/events/internal.h | 1 +
> 4 files changed, 108 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 6bb0c21d6335..8e508f77f42c 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -170,6 +170,12 @@ struct hw_perf_event {
> };
> struct { /* aux / Intel-PT */
> u64 aux_config;
> + /*
> + * For AUX area events, aux_paused cannot be a state
> + * flag because it can be updated asynchronously to
> + * state.
> + */
I would like to name the events calling AUX pausing and resuming as
trigger events.
I am confused for this comment. If I understand correctly, it means
the field 'aux_paused' will not be updated by AUX event itself.
Alternatively, this field will be operated by the triggering events.
> + unsigned int aux_paused;
Combining with the __perf_event_aux_pause() function, should not the
field 'aux_paused' be defined as atomic type? Especially if we
consider the SMP concurrency between the triggering events.
> };
> struct { /* software */
> struct hrtimer hrtimer;
> @@ -294,6 +300,7 @@ struct perf_event_pmu_context;
> #define PERF_PMU_CAP_NO_EXCLUDE 0x0040
> #define PERF_PMU_CAP_AUX_OUTPUT 0x0080
> #define PERF_PMU_CAP_EXTENDED_HW_TYPE 0x0100
> +#define PERF_PMU_CAP_AUX_PAUSE 0x0200
>
> struct perf_output_handle;
>
> @@ -366,6 +373,8 @@ struct pmu {
> #define PERF_EF_START 0x01 /* start the counter when adding */
> #define PERF_EF_RELOAD 0x02 /* reload the counter when starting */
> #define PERF_EF_UPDATE 0x04 /* update the counter when stopping */
> +#define PERF_EF_PAUSE 0x08 /* AUX area event, pause tracing */
> +#define PERF_EF_RESUME 0x10 /* AUX area event, resume tracing */
>
> /*
> * Adds/Removes a counter to/from the PMU, can be done inside a
> @@ -405,6 +414,18 @@ struct pmu {
> *
> * ->start() with PERF_EF_RELOAD will reprogram the counter
> * value, must be preceded by a ->stop() with PERF_EF_UPDATE.
> + *
> + * ->stop() with PERF_EF_PAUSE will stop as simply as possible. Will not
> + * overlap another ->stop() with PERF_EF_PAUSE nor ->start() with
> + * PERF_EF_RESUME.
> + *
> + * ->start() with PERF_EF_RESUME will start as simply as possible but
> + * only if the counter is not otherwise stopped. Will not overlap
> + * another ->start() with PERF_EF_RESUME nor ->stop() with
> + * PERF_EF_PAUSE.
> + *
> + * Notably, PERF_EF_PAUSE/PERF_EF_RESUME *can* be concurrent with other
> + * ->stop()/->start() invocations, just not itself.
> */
> void (*start) (struct perf_event *event, int flags);
> void (*stop) (struct perf_event *event, int flags);
> @@ -1660,6 +1681,13 @@ static inline bool has_aux(struct perf_event *event)
> return event->pmu->setup_aux;
> }
>
> +static inline bool has_aux_action(struct perf_event *event)
> +{
> + return event->attr.aux_sample_size ||
> + event->attr.aux_pause ||
> + event->attr.aux_resume;
> +}
> +
> static inline bool is_write_backward(struct perf_event *event)
> {
> return !!event->attr.write_backward;
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 4842c36fdf80..0524d541d4e3 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -511,7 +511,16 @@ struct perf_event_attr {
> __u16 sample_max_stack;
> __u16 __reserved_2;
> __u32 aux_sample_size;
> - __u32 __reserved_3;
> +
> + union {
> + __u32 aux_action;
> + struct {
> + __u32 aux_start_paused : 1, /* start AUX area tracing paused */
> + aux_pause : 1, /* on overflow, pause AUX area tracing */
> + aux_resume : 1, /* on overflow, resume AUX area tracing */
> + __reserved_3 : 29;
> + };
> + };
>
> /*
> * User provided data if sigtrap=1, passed back to user via
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index aa3450bdc227..e8989db0de18 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2097,7 +2097,7 @@ static void perf_put_aux_event(struct perf_event *event)
>
> static bool perf_need_aux_event(struct perf_event *event)
> {
> - return !!event->attr.aux_output || !!event->attr.aux_sample_size;
> + return event->attr.aux_output || has_aux_action(event);
> }
>
> static int perf_get_aux_event(struct perf_event *event,
> @@ -2122,6 +2122,10 @@ static int perf_get_aux_event(struct perf_event *event,
> !perf_aux_output_match(event, group_leader))
> return 0;
>
> + if ((event->attr.aux_pause || event->attr.aux_resume) &&
> + !(group_leader->pmu->capabilities & PERF_PMU_CAP_AUX_PAUSE))
> + return 0;
> +
> if (event->attr.aux_sample_size && !group_leader->pmu->snapshot_aux)
> return 0;
>
> @@ -7904,6 +7908,49 @@ void perf_prepare_header(struct perf_event_header *header,
> WARN_ON_ONCE(header->size & 7);
> }
>
> +static void __perf_event_aux_pause(struct perf_event *event, bool pause)
> +{
> + if (pause) {
> + if (!event->hw.aux_paused) {
> + event->hw.aux_paused = 1;
> + event->pmu->stop(event, PERF_EF_PAUSE);
As mentioned above, we need to assure the atomiticy for "aux_paused".
> + }
> + } else {
> + if (event->hw.aux_paused) {
> + event->hw.aux_paused = 0;
> + event->pmu->start(event, PERF_EF_RESUME);
> + }
> + }
> +}
> +
> +static void perf_event_aux_pause(struct perf_event *event, bool pause)
> +{
> + struct perf_buffer *rb;
> +
> + if (WARN_ON_ONCE(!event))
> + return;
> +
> + rb = ring_buffer_get(event);
> + if (!rb)
> + return;
Here why we acquire 'rb' pointer'? Seems to me, we should only
operation on a specific AUX event. Thus in this case, we should store
the 'aux_in_pause_resume' field into the structure perf_event.
> +
> + scoped_guard (irqsave) {
> + /*
> + * Guard against self-recursion here. Another event could trip
> + * this same from NMI context.
> + */
> + if (READ_ONCE(rb->aux_in_pause_resume))
> + break;
I understand the race condition between normal flow and NMI flow.
If directly bail out if "aux_in_pause_resume" is true, will it cause
unpaired issue. E.g. one event invokes AUX_PAUSE, and then another
event in the NMI context calls AUX_RESUME, because the NMI context
runs to here and detect the field "aux_in_pause_resume" is true, then
it directly bail out and has no chance to resume the AUX event.
If this is the case, it is better to use read_poll_timeout_atomic() ?
Thanks,
Leo
> +
> + WRITE_ONCE(rb->aux_in_pause_resume, 1);
> + barrier();
> + __perf_event_aux_pause(event, pause);
> + barrier();
> + WRITE_ONCE(rb->aux_in_pause_resume, 0);
> + }
> + ring_buffer_put(rb);
> +}
> +
> static __always_inline int
> __perf_event_output(struct perf_event *event,
> struct perf_sample_data *data,
> @@ -9706,6 +9753,11 @@ static int __perf_event_overflow(struct perf_event *event,
>
> ret = __perf_event_account_interrupt(event, throttle);
>
> + if (event->attr.aux_pause)
> + perf_event_aux_pause(event->aux_event, true);
> + if (event->attr.aux_resume)
> + perf_event_aux_pause(event->aux_event, false);
> +
> if (event->prog && !bpf_overflow_handler(event, data, regs))
> return ret;
>
> @@ -12083,11 +12135,25 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
> }
>
> if (event->attr.aux_output &&
> - !(pmu->capabilities & PERF_PMU_CAP_AUX_OUTPUT)) {
> + (!(pmu->capabilities & PERF_PMU_CAP_AUX_OUTPUT) ||
> + event->attr.aux_pause || event->attr.aux_resume)) {
> err = -EOPNOTSUPP;
> goto err_pmu;
> }
>
> + if (event->attr.aux_pause && event->attr.aux_resume) {
> + err = -EINVAL;
> + goto err_pmu;
> + }
> +
> + if (event->attr.aux_start_paused) {
> + if (!(pmu->capabilities & PERF_PMU_CAP_AUX_PAUSE)) {
> + err = -EOPNOTSUPP;
> + goto err_pmu;
> + }
> + event->hw.aux_paused = 1;
> + }
> +
> if (cgroup_fd != -1) {
> err = perf_cgroup_connect(cgroup_fd, event, attr, group_leader);
> if (err)
> @@ -12883,7 +12949,7 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
> * Grouping is not supported for kernel events, neither is 'AUX',
> * make sure the caller's intentions are adjusted.
> */
> - if (attr->aux_output)
> + if (attr->aux_output || attr->aux_action)
> return ERR_PTR(-EINVAL);
>
> event = perf_event_alloc(attr, cpu, task, NULL, NULL,
> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index 451514442a1b..001b1cbf6f0e 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -51,6 +51,7 @@ struct perf_buffer {
> void (*free_aux)(void *);
> refcount_t aux_refcount;
> int aux_in_sampling;
> + int aux_in_pause_resume;
> void **aux_pages;
> void *aux_priv;
>
> --
> 2.34.1
>
>
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