Date:	Tue, 3 Jun 2014 18:09:55 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tejun Heo <tj@...nel.org>, John Stultz <johnstul@...ibm.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, hughd@...gle.com,
	viresh.kumar@...aro.org, hpa@...or.com, mingo@...nel.org,
	peterz@...radead.org, Mike Frysinger <vapier@...too.org>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH] vmstat: on demand updates from differentials V7

On Thu, May 29, 2014 at 02:56:15PM -0500, Christoph Lameter wrote:
> 
> V6->V7
> - Remove /sysfs support and avoid the large cpumask definition.
> 
> V5->V6:
> - Shepherd thread as a general worker thread. This means
>   that the general mechanism to control worker thread
>   cpu use by Frederic Weisbecker is necessary to
>   restrict the shepherd thread to the cpus not used
>   for low latency tasks. Hopefully that is ready to be
>   merged soon. No need anymore to have a specific
>   cpu be the housekeeper cpu.
> 
> V4->V5:
> - Shepherd thread on a specific cpu (HOUSEKEEPING_CPU).
> - Incorporate Andrew's feedback
> - Work out the races.
> - Make visible which CPUs have stat updates switched off
>   in /sys/devices/system/cpu/stat_off
> 
> V3->V4:
> - Make the shepherd task not deferrable. It runs on the tick cpu
>   anyways. Deferral could get deltas too far out of sync if
>   vmstat operations are disabled for a certain processor.
> 
> V2->V3:
> - Introduce a new tick_get_housekeeping_cpu() function. Not sure
>   if that is exactly what we want but it is a start. Thomas?
> - Migrate the shepherd task if the output of
>   tick_get_housekeeping_cpu() changes.
> - Fixes recommended by Andrew.
> 
> V1->V2:
> - Optimize the need_update check by using memchr_inv.
> - Clean up.
> 
> vmstat workers are used for folding counter differentials into the
> zone, per node and global counters at certain time intervals.
> They currently run at defined intervals on all processors which will
> cause some holdoff for processors that need minimal intrusion by the
> OS.
> 
> The current vmstat_update mechanism depends on a deferrable timer
> firing every other second by default which registers a work queue item
> that runs on the local CPU, with the result that we have 1 interrupt
> and one additional schedulable task on each CPU every 2 seconds
> If a workload indeed causes VM activity or multiple tasks are running
> on a CPU, then there are probably bigger issues to deal with.
> 
> However, some workloads dedicate a CPU for a single CPU bound task.
> This is done in high performance computing, in high frequency
> financial applications, in networking (Intel DPDK, EZchip NPS) and with
> the advent of systems with more and more CPUs over time, this may become
> more and more common to do since when one has enough CPUs
> one cares less about efficiently sharing a CPU with other tasks and
> more about efficiently monopolizing a CPU per task.
> 
> The difference of having this timer firing and workqueue kernel thread
> scheduled per second can be enormous. An artificial test measuring the
> worst case time to do a simple "i++" in an endless loop on a bare metal
> system and under Linux on an isolated CPU with dynticks and with and
> without this patch, have Linux match the bare metal performance
> (~700 cycles) with this patch and loose by couple of orders of magnitude
> (~200k cycles) without it[*].  The loss occurs for something that just
> calculates statistics. For networking applications, for example, this
> could be the difference between dropping packets or sustaining line rate.
> 
> Statistics are important and useful, but it would be great if there
> would be a way to not cause statistics gathering produce a huge
> performance difference. This patche does just that.
> 
> This patch creates a vmstat shepherd worker that monitors the
> per cpu differentials on all processors. If there are differentials
> on a processor then a vmstat worker local to the processors
> with the differentials is created. That worker will then start
> folding the diffs in regular intervals. Should the worker
> find that there is no work to be done then it will make the shepherd
> worker monitor the differentials again.
> 
> With this patch it is possible then to have periods longer than
> 2 seconds without any OS event on a "cpu" (hardware thread).
> 
> Reviewed-by: Gilad Ben-Yossef <gilad@...yossef.com>
> Signed-off-by: Christoph Lameter <cl@...ux.com>

So after the cpumask_var_t conversion I have no other concern except
perhaps that the scan may bring some overhead on workloads that don't
care about isolation. You might want to make it optional. But I let you
check that.

And I can't judge much the -mm internal changes. But other than that, it looks good to me.

Thanks.
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