linux-kernel - Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

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Date:   Mon, 20 Mar 2023 19:25:55 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Huacai Chen <chenhuacai@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, x86@...nel.org,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

On Mon 20-03-23 15:03:32, Marcelo Tosatti wrote:
> This patch series addresses the following two problems:
> 
> 1. A customer provided evidence indicating that a process
>    was stalled in direct reclaim:
> 
This is addressed by the trivial patch 1.

[...]
>  2. With a task that busy loops on a given CPU,
>     the kworker interruption to execute vmstat_update
>     is undesired and may exceed latency thresholds
>     for certain applications.

Yes it can but why does that matter?

> By having vmstat_shepherd flush the per-CPU counters to the
> global counters from remote CPUs.
> 
> This is done using cmpxchg to manipulate the counters,
> both CPU locally (via the account functions),
> and remotely (via cpu_vm_stats_fold).
> 
> Thanks to Aaron Tomlin for diagnosing issue 1 and writing
> the initial patch series.
> 
> 
> Performance details for the kworker interruption:
> 
> oslat   1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
> oslat   1094.456971: workqueue_queue_work: ... function=vmstat_update ...
> oslat   1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
> kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...
>  
> The example above shows an additional 7us for the
> 
>         oslat -> kworker -> oslat
> 
> switches. In the case of a virtualized CPU, and the vmstat_update
> interruption in the host (of a qemu-kvm vcpu), the latency penalty
> observed in the guest is higher than 50us, violating the acceptable
> latency threshold for certain applications.

I do not think we have ever promissed any specific latency guarantees
for vmstat. These are statistics have been mostly used for debugging
purposes AFAIK. I am not aware of any specific user space use case that
would be latency sensitive. Your changelog doesn't go into details there
either.

[...]
>  mm/vmstat.c                         |  440 +++++++++++++++++++++++++++++++++++++++++++++++------------------------------

This requires much more detailed story why we really need that.
-- 
Michal Hocko
SUSE Labs