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SubjectRe: [PATCHSET 0/6] perf lock: Add contention subcommand (v1)
Hi Ingo,

On Wed, Aug 3, 2022 at 2:51 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Namhyung Kim <namhyung@kernel.org> wrote:
>
> > Hello,
> >
> > It's to add a new subcommand 'contention' (shortly 'con') to perf lock.
> >
> > The new subcommand is to handle the new lock:contention_{begin,end}
> > tracepoints and shows lock type and caller address like below:
> >
> > $ perf lock contention
> > contended total wait max wait avg wait type caller
> >
> > 238 1.41 ms 29.20 us 5.94 us spinlock update_blocked_averages+0x4c
> > 1 902.08 us 902.08 us 902.08 us rwsem:R do_user_addr_fault+0x1dd
> > 81 330.30 us 17.24 us 4.08 us spinlock _nohz_idle_balance+0x172
> > 2 89.54 us 61.26 us 44.77 us spinlock do_anonymous_page+0x16d
> > 24 78.36 us 12.27 us 3.27 us mutex pipe_read+0x56
> > 2 71.58 us 59.56 us 35.79 us spinlock __handle_mm_fault+0x6aa
> > 6 25.68 us 6.89 us 4.28 us spinlock do_idle+0x28d
> > 1 18.46 us 18.46 us 18.46 us rtmutex exec_fw_cmd+0x21b
> > 3 15.25 us 6.26 us 5.08 us spinlock tick_do_update_jiffies64+0x2c
> > ...
>
> Wouldn't it also be useful to display a lock contention percentage value,
> the ratio of fastpath vs. contended/wait events?
>
> That's usually the first-approximation metric to see how contended
> different locks are, and the average wait time quantifies it.

Yeah, that'd be nice to have. But it requires some action in the fast path
which I don't want because I'd like to use this in production. So these
new tracepoints were added only in the slow path.

Instead, I think we can display the ratio of (total) contended time vs.
wall clock time. What do you think?

Thanks,
Namhyung

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