linux-kernel - Re: [PATCH bpf-next 1/3] perf: enable branch record for software events

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Date:   Wed, 25 Aug 2021 14:09:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        mingo@...hat.com, kernel-team@...com
Subject: Re: [PATCH bpf-next 1/3] perf: enable branch record for software
 events

On Mon, Aug 23, 2021 at 11:01:55PM -0700, Song Liu wrote:

>  arch/x86/events/intel/core.c |  5 ++++-
>  arch/x86/events/intel/lbr.c  | 12 ++++++++++++
>  arch/x86/events/perf_event.h |  2 ++
>  include/linux/perf_event.h   | 33 +++++++++++++++++++++++++++++++++
>  kernel/events/core.c         | 28 ++++++++++++++++++++++++++++
>  5 files changed, 79 insertions(+), 1 deletion(-)

No PowerPC support :/

> +void intel_pmu_snapshot_branch_stack(void)
> +{
> +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> +
> +	intel_pmu_lbr_disable_all();
> +	intel_pmu_lbr_read();
> +	memcpy(this_cpu_ptr(&perf_branch_snapshot_entries), cpuc->lbr_entries,
> +	       sizeof(struct perf_branch_entry) * x86_pmu.lbr_nr);
> +	*this_cpu_ptr(&perf_branch_snapshot_size) = x86_pmu.lbr_nr;
> +	intel_pmu_lbr_enable_all(false);
> +}

Still has the layering violation and issues vs PMI.

> +#ifdef CONFIG_HAVE_STATIC_CALL
> +DECLARE_STATIC_CALL(perf_snapshot_branch_stack,
> +		    perf_default_snapshot_branch_stack);
> +#else
> +extern void (*perf_snapshot_branch_stack)(void);
> +#endif

That's weird, static call should work unconditionally, and fall back to
a regular function pointer exactly like you do here. Search for:
"Generic Implementation" in include/linux/static_call.h

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 011cc5069b7ba..b42cc20451709 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c

> +#ifdef CONFIG_HAVE_STATIC_CALL
> +DEFINE_STATIC_CALL(perf_snapshot_branch_stack,
> +		   perf_default_snapshot_branch_stack);
> +#else
> +void (*perf_snapshot_branch_stack)(void) = perf_default_snapshot_branch_stack;
> +#endif

Idem.

Something like:

DEFINE_STATIC_CALL_NULL(perf_snapshot_branch_stack, void (*)(void));

with usage like: static_call_cond(perf_snapshot_branch_stack)();

Should unconditionally work.

> +int perf_read_branch_snapshot(void *buf, size_t len)
> +{
> +	int cnt;
> +
> +	memcpy(buf, *this_cpu_ptr(&perf_branch_snapshot_entries),
> +	       min_t(u32, (u32)len,
> +		     sizeof(struct perf_branch_entry) * MAX_BRANCH_SNAPSHOT));
> +	cnt =  *this_cpu_ptr(&perf_branch_snapshot_size);
> +
> +	return (cnt > 0) ? cnt : -EOPNOTSUPP;
> +}

Doesn't seem used at all..