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

[Peter Zijlstra]

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 ;-)