linux-kernel - Re: [PATCH v6 bpf-next 1/3] perf: enable branch record for software events

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Date:   Fri, 10 Sep 2021 20:40:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kjain@...ux.ibm.com" <kjain@...ux.ibm.com>,
        Kernel Team <Kernel-team@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH v6 bpf-next 1/3] perf: enable branch record for software
 events

On Fri, Sep 10, 2021 at 06:27:36PM +0000, Song Liu wrote:

> This works great and saves 3 entries! We have the following now:

Yay!

> ID: 0 from bpf_get_branch_snapshot+18 to intel_pmu_snapshot_branch_stack+0

is unavoidable, we need to end up in intel_pmu_snapshot_branch_stack()
eventually.

> ID: 1 from __brk_limit+477143934 to bpf_get_branch_snapshot+0

could be elided by having the JIT emit the call to
intel_pmu_snapshot_branch_stack directly, instead of laundering it
through that helper I suppose.

> ID: 2 from __brk_limit+477192263 to __brk_limit+477143880  # trampoline 
> ID: 3 from __bpf_prog_enter+34 to __brk_limit+477192251

-ENOCLUE

> ID: 4 from migrate_disable+60 to __bpf_prog_enter+9
> ID: 5 from __bpf_prog_enter+4 to migrate_disable+0

I suppose we can reduce that to a single branch if we inline
migrate_disable() here, that thing unfortunately needs one branch
itself.

> ID: 6 from bpf_testmod_loop_test+20 to __bpf_prog_enter+0

And this is the first branch out of the test program, giving 7 entries
now, of which we can remove at least 2 more with a bit of elbow greace,
right?

> ID: 7 from bpf_testmod_loop_test+20 to bpf_testmod_loop_test+13
> ID: 8 from bpf_testmod_loop_test+20 to bpf_testmod_loop_test+13
> 
> I will fold this in and send v7. 

Excellent.