linux-kernel - Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)

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Date:	Sun, 24 Jan 2016 03:46:21 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable
 again and shut down on idle)

On Sat, 2016-01-23 at 18:33 -0600, Christoph Lameter wrote:
> On Sat, 23 Jan 2016, Mike Galbraith wrote:
> 
> > While you're fixing that commit up, can you perhaps find a better home
> > for quiet_vmstat()?  It not only munches cycles when switching cross
> > -core mightily, for -rt it injects a sleeping lock into the idle task.
> 
> Not sure what you are talking about. No sleeping locks are used in
> quiet_vmstat() nor does it switch across cores. It would be broken if it
> would do so.

By switching cross-core, I'm referring to scheduling of communicating
tasks.

The perf top snippet...

    12.89%  [kernel]       [k] refresh_cpu_vm_stats.isra.12
     4.75%  [kernel]       [k] __schedule                  
     4.70%  [kernel]       [k] mutex_unlock                
     3.14%  [kernel]       [k] __switch_to

... was pipe-test, an unrealistic microbench, but the same will happen
at lower frequency in the real world.

Here's the sleeping lock for -rt:

[    2.279582] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.5.0-rt3 #7
[    2.280444] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[    2.281316]  ffff88040b00d640 ffff88040b01fe10 ffffffff812d20e2 0000000000000000
[    2.282202]  ffff88040b01fe30 ffffffff81081095 ffff88041ec4cee0 ffff88041ec501e0
[    2.283073]  ffff88040b01fe48 ffffffff815ff910 ffff88041ec4cee0 ffff88040b01fe88
[    2.283941] Call Trace:
[    2.284797]  [<ffffffff812d20e2>] dump_stack+0x49/0x67
[    2.285658]  [<ffffffff81081095>] ___might_sleep+0xf5/0x180
[    2.286521]  [<ffffffff815ff910>] rt_spin_lock+0x20/0x50
[    2.287382]  [<ffffffff81075919>] try_to_grab_pending+0x69/0x240
[    2.288239]  [<ffffffff81075b16>] cancel_delayed_work+0x26/0xe0
[    2.289094]  [<ffffffff8115ec05>] quiet_vmstat+0x75/0xa0
[    2.289949]  [<ffffffff8109ab38>] cpu_idle_loop+0x38/0x3e0
[    2.290800]  [<ffffffff8109aef3>] cpu_startup_entry+0x13/0x20
[    2.291647]  [<ffffffff81036164>] start_secondary+0x114/0x140