linux-kernel - Re: [PATCH] sched: remove sched_find_first

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Date:   Mon, 15 May 2017 19:17:08 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Akinobu Mita <mita@...aclelinux.com>,
        Mike Galbraith <efault@....de>,
        Yury Norov <ynorov@...iumnetworks.com>
Subject: Re: [PATCH] sched: remove sched_find_first_bit()

On Mon, May 15, 2017 at 06:06:18PM +0200, Arnd Bergmann wrote:
> On Mon, May 15, 2017 at 5:47 PM, Yury Norov <ynorov@...iumnetworks.com> wrote:
> > On Sun, May 14, 2017 at 08:09:17PM +0200, Ingo Molnar wrote:
> >>
> >> * Yury Norov <ynorov@...iumnetworks.com> wrote:
> >>
> >> > sched_find_first_bit() is in fact the unrolled version of
> >> > find_first_bit(), which is theoretically faster in some cases.
> >> > But in the kernel it is called only in couple places in
> >> > kernel/sched/rt.c, and both of them are not looking like hot
> >> > paths [...]
> >>
> >> They are in terms of scheduling: pick_next_rt_entity() is in the RT scheduling
> >> fastpath.
> >
> > Sorry that. I'll be more specific. I was only saying that pick_next_rt_entity()
> > is big enough to feel any difference, and it's still true for me. Please
> > forget about hot paths.
> >
> >> Which makes me just suspicious of how careful this patch really is:
> >>
> >> > that will doubtly achieve measurable benefit from using unrolled version of
> >> > find_first_bit() - there's no hard loops, and the execution path is not really
> >> > short.
> >>
> >> ... that's really just handwaving. Numbers please.
> >
> > I use qemu running arm64 as my testing environment. It's not the best
> > for performance measurements, but allows estimate something... So,
> >
> > This patch shows the time (in cycles) that kernel spends running the
> > pick_next_rt_entity() code:
> >
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index f04346329204..7c6194e30230 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -1529,6 +1529,8 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >         struct task_struct *p;
> >         struct rt_rq *rt_rq = &rq->rt;
> >
> > +       u64 cycles = get_cycles();
> > +
> >         if (need_pull_rt_task(rq, prev)) {
> >                 /*
> >                  * This is OK, because current is on_cpu, which avoids it being
> > @@ -1568,6 +1570,8 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >
> >         queue_push_tasks(rq);
> >
> > +       pr_err("cycles: %lld\n", get_cycles() - cycles);
> > +
> >         return p;
> >  }
> >
> > I collected about 700 results in dmesg, and took 600 fastest.
> > For the vanilla kernel, the average value is 368, and for patched
> > kernel it is 388. It's 5% slower. But the standard deviation is
> > really big for both series' - 131 and 106 cycles respectively, which
> > is ~ 30%. And so, my conclusion is: there's no benefit in using
> > sched_find_first_bit() comparing to find_first_bit().
> >
> > I also think that sched_find_first_bit() may be faster that find_first_bit()
> > because it's inlined in the caller. We can do so for find_first_bit() if
> > it takes small sizes at compile time, and so all parts of kernel will
> > use fast find_first_bit, not only sched.
> 
> I suspect the first step would be to 'select GENERIC_FIND_FIRST_BIT'
> on ARM64, which should already improve the performance for those
> files that never call the 'next' variants.
> 
> Adding an inline version of find_first_{,zero_}bit could also help, but
> is harder to quantify.
> 
>         Arnd

I checked again, and in fact I measured on top of this patch:
https://lkml.org/lkml/2017/5/13/137
So find_first_bit is already enabled.