2024-04-25 22:18:47

by Yabin Cui

Subject: [PATCH] perf/core: Trim dyn_size if raw data is absent

Currently, perf_tp_event() always allocates space for raw sample data,
even when the PERF_SAMPLE_RAW flag is not set. This leads to unused
spaces within generated sample records.

This patch reduces dyn_size when PERF_SAMPLE_RAW is not present,
ensuring sample records use only the necessary amount of space.

Fixes: 0a9081cf0a11 ("perf/core: Add perf_sample_save_raw_data() helper")
Signed-off-by: Yabin Cui <[email protected]>
---
kernel/events/core.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 724e6d7e128f..d68ecdc264d3 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7688,6 +7688,10 @@ void perf_prepare_sample(struct perf_sample_data *data,
data->raw = NULL;
data->dyn_size += sizeof(u64);
data->sample_flags |= PERF_SAMPLE_RAW;
+ } else if ((data->sample_flags & ~sample_type) & PERF_SAMPLE_RAW) {
+ data->dyn_size -= data->raw->size + sizeof(u32);
+ data->raw = NULL;
+ data->sample_flags &= ~PERF_SAMPLE_RAW;
}

if (filtered_sample_type & PERF_SAMPLE_BRANCH_STACK) {
--
2.44.0.769.g3c40516874-goog

2024-04-29 22:00:10

by Namhyung Kim

Subject: Re: [PATCH] perf/core: Trim dyn_size if raw data is absent

Hello Yabin,

CC-ing the bpf list.

On Thu, Apr 25, 2024 at 3:05 PM Yabin Cui <[email protected]> wrote:
>
> Currently, perf_tp_event() always allocates space for raw sample data,
> even when the PERF_SAMPLE_RAW flag is not set. This leads to unused
> spaces within generated sample records.
>
> This patch reduces dyn_size when PERF_SAMPLE_RAW is not present,
> ensuring sample records use only the necessary amount of space.

Right, it seems bpf-output and tracepoint events set the flags without
checking PERF_SAMPLE_RAW. Can you fix the callsites instead?
Or we can add perf_event argument to perf_sample_save_raw_data()
and check the flag inside.

We might reject the output data when it's not opened with the flag.
But I'm afraid it might break some existing BPF programs.

Thanks,
Namhyung

>
> Fixes: 0a9081cf0a11 ("perf/core: Add perf_sample_save_raw_data() helper")
> Signed-off-by: Yabin Cui <[email protected]>
> ---
> kernel/events/core.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 724e6d7e128f..d68ecdc264d3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7688,6 +7688,10 @@ void perf_prepare_sample(struct perf_sample_data *data,
> data->raw = NULL;
> data->dyn_size += sizeof(u64);
> data->sample_flags |= PERF_SAMPLE_RAW;
> + } else if ((data->sample_flags & ~sample_type) & PERF_SAMPLE_RAW) {
> + data->dyn_size -= data->raw->size + sizeof(u32);
> + data->raw = NULL;
> + data->sample_flags &= ~PERF_SAMPLE_RAW;
> }
>
> if (filtered_sample_type & PERF_SAMPLE_BRANCH_STACK) {
> --
> 2.44.0.769.g3c40516874-goog
>

2024-05-02 00:10:48

by Yabin Cui

Subject: Re: [PATCH] perf/core: Trim dyn_size if raw data is absent

Hi Namyung,

Thanks for reviewing the patch! Fixing the callsites is a better idea.
I have sent a v2 patch with name [PATCH v2] perf/core: Save raw sample
data conditionally based on sample type.
Rejecting tracepoint events without PERF_SAMPLE_RAW will break my use
case in Android. So I hope we don't do that.

Thanks,
Yabin






On Mon, Apr 29, 2024 at 2:59 PM Namhyung Kim <[email protected]> wrote:
>
> Hello Yabin,
>
> CC-ing the bpf list.
>
> On Thu, Apr 25, 2024 at 3:05 PM Yabin Cui <[email protected]> wrote:
> >
> > Currently, perf_tp_event() always allocates space for raw sample data,
> > even when the PERF_SAMPLE_RAW flag is not set. This leads to unused
> > spaces within generated sample records.
> >
> > This patch reduces dyn_size when PERF_SAMPLE_RAW is not present,
> > ensuring sample records use only the necessary amount of space.
>
> Right, it seems bpf-output and tracepoint events set the flags without
> checking PERF_SAMPLE_RAW. Can you fix the callsites instead?
> Or we can add perf_event argument to perf_sample_save_raw_data()
> and check the flag inside.
>
> We might reject the output data when it's not opened with the flag.
> But I'm afraid it might break some existing BPF programs.
>
> Thanks,
> Namhyung
>
> >
> > Fixes: 0a9081cf0a11 ("perf/core: Add perf_sample_save_raw_data() helper")
> > Signed-off-by: Yabin Cui <[email protected]>
> > ---
> > kernel/events/core.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 724e6d7e128f..d68ecdc264d3 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -7688,6 +7688,10 @@ void perf_prepare_sample(struct perf_sample_data *data,
> > data->raw = NULL;
> > data->dyn_size += sizeof(u64);
> > data->sample_flags |= PERF_SAMPLE_RAW;
> > + } else if ((data->sample_flags & ~sample_type) & PERF_SAMPLE_RAW) {
> > + data->dyn_size -= data->raw->size + sizeof(u32);
> > + data->raw = NULL;
> > + data->sample_flags &= ~PERF_SAMPLE_RAW;
> > }
> >
> > if (filtered_sample_type & PERF_SAMPLE_BRANCH_STACK) {
> > --
> > 2.44.0.769.g3c40516874-goog
> >