linux-kernel - Re: [PATCH] mm/vmstat: Defer the refresh_zone_stat

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Message-ID: <20240828053737.GA22322@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date: Tue, 27 Aug 2024 22:37:37 -0700
From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, ssengar@...rosoft.com,
	wei.liu@...nel.org
Subject: Re: [PATCH] mm/vmstat: Defer the refresh_zone_stat_thresholds after
 all CPUs bringup

On Fri, Aug 23, 2024 at 08:32:43AM -0700, Christoph Lameter (Ampere) wrote:
> On Mon, 8 Jul 2024, Saurabh Singh Sengar wrote:
> 
> > > > Calling this for each CPU is expensive when there are large number
> > > > of CPUs along with multiple NUMAs. Fix this by deferring
> > > > refresh_zone_stat_thresholds to be called later at once when all the
> > > > secondary CPUs are up. Also, register the DYN hooks to keep the
> > > > existing hotplug functionality intact.
> > > >
> > >
> > > Seems risky - we'll now have online CPUs which have unintialized data,
> > > yes?  What assurance do we have that this data won't be accessed?
> >
> > I understand that this data is only accessed by userspace tools, and they can
> > only access it post late_initcall. Please let me know if there are any other
> > cases, I will look to address them.
> 
> stat_threshold is used in all statistics functions that modify VM
> counters. It is core to the functioning of the VM statistics.
> 
> However, if the threshold is zero (not initialized) then the VM counter
> handling will simply be less effective because it will not do the per cpu
> counter diffs anymore. This may increase contention and eat up the benefit
> you are getting from deferring the calculation of the threshholds.

Christoph,

Thank you for your review. I would like to gain a better understanding of how
to measure contention. Could you please inform me if there is any recommended
method for doing so?

In my testing, this patch has resulted in a significant improvement in boot time.

> 
> What may be more promising is to make it possible to calculate the
> threshholds per cpu instead of recalculating the thresholds for every zone
> again and again.

I am happy to explore alternatives, can you please share more details around
this approach. Are you referring to avoiding the repetition of the calculation
below?

mem = zone_managed_pages(zone) >> (27 - PAGE_SHIFT);

- Saurabh