Re: [Bug #15044] Much higher wakeups for "<kernel IPI> : Rescheduling interrupts" since 2.6.32.2

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Monday 15 February 2010, Mike Galbraith wrote:
> On Mon, 2010-02-15 at 00:38 +0100, Rafael J. Wysocki wrote:
> > This message has been generated automatically as a part of a summary report
> > of recent regressions.
> > 
> > The following bug entry is on the current list of known regressions
> > from 2.6.32.  Please verify if it still should be listed and let the tracking team
> > know (either way).
> > 
> > 
> > Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=15044
> > Subject		: Much higher wakeups for "<kernel IPI> : Rescheduling interrupts" since 2.6.32.2
> > Submitter	: Roman Mamedov <roman@rm.pp.ru>
> > Date		: 2010-01-11 02:58 (35 days old)
> > First-Bad-Commit: http://git.kernel.org/git/linus/a1f84a3ab8e002159498814eaa7e48c33752b04b
> 
> I don't know that this should be carried as a regression.
> 
> Yes, the code in question increases cross cpu wakeups, but that's the
> entire point.  If there is any overlap in execution larger than the cost
> of running a scheduler on another core, that time can be converted to
> throughput.
> 
> Tip AF_UNIX lmbench numbers show that throughput gain being realized,
> TCP numbers below that (x) show what can be had for apps which do a lot
> of what that microbenchmark does, given a tiny enabler patchlet.
> 
> *Local* Communication bandwidths in MB/s - bigger is better
> -----------------------------------------------------------------------------
> Host                OS  Pipe AF    TCP  File   Mmap  Bcopy  Bcopy  Mem   Mem
>                              UNIX      reread reread (libc) (hand) read write
> --------- ------------- ---- ---- ---- ------ ------ ------ ------ ---- -----
> marge      2.6.31.9-smp 2853 2923 1132 2829.3 4761.9 1235.0 1234.4 4472 1683.
> marge      2.6.31.9-smp 2839 2921 1141 2846.5 4779.8 1242.5 1235.9 4455 1684.
> marge      2.6.31.9-smp 2838 2935 751. 2838.5 4820.0 1243.6 1235.0 4472 1684.
> marge    2.6.33-tip-smp 3057 5166 859. 2760.2 4827.8 1481.1 1466.1 4499 1811.
> marge    2.6.33-tip-smp 1796 5165 1257 2748.6 4817.4 1481.1 1464.8 4487 1806.
> marge    2.6.33-tip-smp 3055 5175 1262 2763.4 4812.4 1483.9 1462.7 4477 1810.
> marge   2.6.33-tip-smpx 3063 5140 2940 2811.1 4740.0 1235.8 1237.0 4433 1673.
> marge   2.6.33-tip-smpx 3065 5205 2945 2836.3 4794.4 1243.6 1233.7 4293 1686.
> marge   2.6.33-tip-smpx 3058 5181 2940 2785.4 4700.2 1243.9 1234.5 4415 1682.
> 
> (1. tip memory numbers are phase-of-moon anomaly.. irrelevant here)
> (2. pipe numbers are only possible with pipe buffer increase patch in
> tip.  Often, pipes are truly synchronous, so waking cross CPU is small
> loss.  In tip, it's a win because of optimistic mutex spin.. context
> switch cost is converted to throughput.  That throughput gain also
> cannot be had if you don't do the cross cpu wakeup to get the ball
> rolling.  The code in question is acting as enabler for spintex.)
> 
> So yeah, the code in question _will_ cause more cross CPU wakeups, and
> it _may_ cost power.  It may _save_ power by getting the job done more
> efficiently.  Dunno.
> 
> Regression?  Depends on what you're measuring.

OK, so I'm going to close this report as "documented", since this is
intentional behavior.

Thanks,
Rafael