Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Qais Yousef <qais.yousef@arm.com>]

On 05/29/20 11:08, Mel Gorman wrote:
> On Thu, May 28, 2020 at 06:11:12PM +0200, Peter Zijlstra wrote:
> > > FWIW, I think you're referring to Mel's notice in OSPM regarding the overhead.
> > > Trying to see what goes on in there.
> > 
> > Indeed, that one. The fact that regular distros cannot enable this
> > feature due to performance overhead is unfortunate. It means there is a
> > lot less potential for this stuff.
> 
> During that talk, I was a vague about the cost, admitted I had not looked
> too closely at mainline performance and had since deleted the data given
> that the problem was first spotted in early April. If I heard someone
> else making statements like I did at the talk, I would consider it a bit
> vague, potentially FUD, possibly wrong and worth rechecking myself. In
> terms of distributions "cannot enable this", we could but I was unwilling
> to pay the cost for a feature no one has asked for yet. If they had, I
> would endevour to put it behind static branches and disable it by default
> (like what happened for PSI). I was contacted offlist about my comments
> at OSPM and gathered new data to respond properly. For the record, here
> is an editted version of my response;

I had this impression too that's why I had a rather humble attempt.

[...]

> # Event 'cycles:ppp'
> #
> # Baseline  Delta Abs  Shared Object             Symbol
> # ........  .........  ........................  ..............................................
> #
>      9.59%     -2.87%  [kernel.vmlinux]          [k] poll_idle
>      0.19%     +1.85%  [kernel.vmlinux]          [k] activate_task
>                +1.17%  [kernel.vmlinux]          [k] dequeue_task
>                +0.89%  [kernel.vmlinux]          [k] update_rq_clock.part.73
>      3.88%     +0.73%  [kernel.vmlinux]          [k] try_to_wake_up
>      3.17%     +0.68%  [kernel.vmlinux]          [k] __schedule
>      1.16%     -0.60%  [kernel.vmlinux]          [k] __update_load_avg_cfs_rq
>      2.20%     -0.54%  [kernel.vmlinux]          [k] resched_curr
>      2.08%     -0.29%  [kernel.vmlinux]          [k] _raw_spin_lock_irqsave
>      0.44%     -0.29%  [kernel.vmlinux]          [k] cpus_share_cache
>      1.13%     +0.23%  [kernel.vmlinux]          [k] _raw_spin_lock_bh
> 
> A lot of the uclamp functions appear to be inlined so it is not be
> particularly obvious from a raw profile but it shows up in the annotated
> profile in activate_task and dequeue_task for example. In the case of
> dequeue_task, uclamp_rq_dec_id() is extremely expensive according to the
> annotated profile.
> 
> I'm afraid I did not dig into this deeply once I knew I could just disable
> it even within the distribution.

Could by any chance the vmlinux (with debug symbols hopefully) and perf.dat are
still lying around to share?

I could send you a link to drop them somewhere.

Thanks

--
Qais Yousef