Re: [PATCH bpf-next v8 2/3] perf: Refactor get_perf_callchain

[Andrii Nakryiko]

On Wed, Jan 28, 2026 at 1:10 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Jan 26, 2026 at 03:43:30PM +0800, Tao Chen wrote:
> > From BPF stack map, we want to ensure that the callchain buffer
> > will not be overwritten by other preemptive tasks and we also aim
> > to reduce the preempt disable interval, Based on the suggestions from Peter
> > and Andrrii, export new API __get_perf_callchain and the usage scenarios
> > are as follows from BPF side:
> >
> > preempt_disable()
> > entry = get_callchain_entry()
> > preempt_enable()
> > __get_perf_callchain(entry)
> > put_callchain_entry(entry)
>
> That makes no sense, this means any other task on that CPU is getting
> screwed over.

Yes, unfortunately, but unless we dynamically allocate new entry each
time and/or keep per-current entry cached there isn't much choice we
have here, no?

Maybe that's what we have to do, honestly, because
get_perf_callchain() usage we have right now from sleepable BPF isn't
great no matter with or without changes in this patch set.

>
> Why are you worried about the preempt_disable() here? If this were an
> interrupt context we'd still do that unwind -- but then with IRQs
> disabled.

Because __bpf_get_stack() from kernel/bpf/stackmap.c can be called
from sleepable/faultable context and also we can do a rather expensive
build ID resolution (either in sleepable or not, which only changes if
build ID parsing logic waits for file backed pages to be paged in or
not).