linux-kernel - Re: find_busiest_group using lots of CPU

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Date:	Tue, 6 Oct 2009 13:56:56 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: find_busiest_group using lots of CPU

On Tue, Oct 06 2009, Ingo Molnar wrote:
> 
> * Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > On Tue, Oct 06 2009, Jens Axboe wrote:
> > > On Mon, Oct 05 2009, Peter Zijlstra wrote:
> > > > On Wed, 2009-09-30 at 10:18 +0200, Jens Axboe wrote:
> > > > > Hi,
> > > > > 
> > > > > I stuffed a few more SSDs into my text box. Running a simple workload
> > > > > that just does streaming reads from 10 processes (throughput is around
> > > > > 2.2GB/sec), find_busiest_group() is using > 10% of the CPU time. This is
> > > > > a 64 thread box.
> > > > > 
> > > > > The top two profile entries are:
> > > > > 
> > > > >     10.86%      fio  [kernel]                [k] find_busiest_group
> > > > >                 |          
> > > > >                 |--99.91%-- thread_return
> > > > >                 |          io_schedule
> > > > >                 |          sys_io_getevents
> > > > >                 |          system_call_fastpath
> > > > >                 |          0x7f4b50b61604
> > > > >                 |          |          
> > > > >                 |           --100.00%-- td_io_getevents
> > > > >                 |                     io_u_queued_complete
> > > > >                 |                     thread_main
> > > > >                 |                     run_threads
> > > > >                 |                     main
> > > > >                 |                     __libc_start_main
> > > > >                  --0.09%-- [...]
> > > > > 
> > > > >      5.78%      fio  [kernel]                [k] cpumask_next_and
> > > > >                 |          
> > > > >                 |--67.21%-- thread_return
> > > > >                 |          io_schedule
> > > > >                 |          sys_io_getevents
> > > > >                 |          system_call_fastpath
> > > > >                 |          0x7f4b50b61604
> > > > >                 |          |          
> > > > >                 |           --100.00%-- td_io_getevents
> > > > >                 |                     io_u_queued_complete
> > > > >                 |                     thread_main
> > > > >                 |                     run_threads
> > > > >                 |                     main
> > > > >                 |                     __libc_start_main
> > > > >                 |          
> > > > >                  --32.79%-- find_busiest_group
> > > > >                            thread_return
> > > > >                            io_schedule
> > > > >                            sys_io_getevents
> > > > >                            system_call_fastpath
> > > > >                            0x7f4b50b61604
> > > > >                            |          
> > > > >                             --100.00%-- td_io_getevents
> > > > >                                       io_u_queued_complete
> > > > >                                       thread_main
> > > > >                                       run_threads
> > > > >                                       main
> > > > >                                       __libc_start_main
> > > > > 
> > > > > This is with SCHED_DEBUG=y and SCHEDSTATS=y enabled, I just tried with
> > > > > both disabled but that yields the same result (well actually worse, 22%
> > > > > spent in there. dunno if that's normal "fluctuation"). GROUP_SCHED is
> > > > > not set. This seems way excessive!
> > > > 
> > > > io_schedule() straight into find_busiest_group() leads me to think this
> > > > could be SD_BALANCE_NEWIDLE, does something like:
> > > > 
> > > > for i in /proc/sys/kernel/sched_domain/cpu*/domain*/flags; 
> > > > do 
> > > > 	val=`cat $i`; echo $((val & ~0x02)) > $i; 
> > > > done
> > > > 
> > > > [ assuming SCHED_DEBUG=y ]
> > > > 
> > > > Cure things?
> > > 
> > > I can try, as mentioned it doesn't look any better with SCHED_DEBUG=n
> > 
> > It does, it's gone from the profiles.
> 
> Peter mentioned SCHED_DEBUG=y to have 
> /proc/sys/kernel/sched_domain/cpu*/domain*/flags available.

In case it wasn't clear, what I did was run the 'for...' from Peter
(with a kernel with SCHED_DEBUG=y) and it cured the "problem". Peter
asked whether it cured things, to which I replied "It does, it's gone
from the profiles".

-- 
Jens Axboe

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