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

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Date:   Thu, 23 Mar 2023 08:51:14 +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 Wed 22-03-23 11:20:55, Marcelo Tosatti wrote:
> On Wed, Mar 22, 2023 at 02:35:20PM +0100, Michal Hocko wrote:
[...]
> > > "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."
> > 
> > Yes, I have seen that but it doesn't really give a wider context to
> > understand why those numbers matter.
> 
> OK.
> 
> "In the case of RAN, a MAC scheduler with TTI=1ms, this causes >100us
> interruption observed in a guest (which is above the safety
> threshold for this application)."
> 
> Is that OK?

This might be a sufficient information for somebody familiar with the
matter (not me). So no, not enough. We need to hear a more complete
story. 

-- 
Michal Hocko
SUSE Labs