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

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Message-ID: <20210910190804.GS4323@worktop.programming.kicks-ass.net>
Date:   Fri, 10 Sep 2021 21:08:04 +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 07:00:08PM +0000, Song Liu wrote:

> Hmm.. not really. We call migrate_disable() before entering the BPF program. 
> And the helper calls snapshot_branch_stack() inside the BPF program. To move
> it to before migrate_disable(), we will have to add a "whether to snapshot
> branch stack" check before entering the BPF program. This check, while is
> cheap, is added to all BPF programs on this hook, even when the program does 
> not use snapshot at all. So we would rather keep all logic inside the helper, 
> and not touch the common path. 

Moo :/ Because I also really don't want to expose struct rq, it's
currently nicely squirelled away in kernel/sched/ and doesn't get
anywhere near include/.

A well, maybe we can do something clever with migrate_disable() itself.
I'll put it on this endless todo list ;-)