linux-kernel - Re: [PATCH v1 0/2] mm: migrate: vm event counter for hugepage migration

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Date:   Thu, 12 Apr 2018 09:47:54 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Zi Yan <zi.yan@...t.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 0/2] mm: migrate: vm event counter for hugepage
 migration

On Thu 12-04-18 07:40:41, Naoya Horiguchi wrote:
> On Thu, Apr 12, 2018 at 08:18:59AM +0200, Michal Hocko wrote:
> > On Wed 11-04-18 17:09:25, Naoya Horiguchi wrote:
> > > Hi everyone,
> > > 
> > > I wrote patches introducing separate vm event counters for hugepage migration
> > > (both for hugetlb and thp.)
> > > Hugepage migration is different from normal page migration in event frequency
> > > and/or how likely it succeeds, so maintaining statistics for them in mixed
> > > counters might not be helpful both for develors and users.
> > 
> > This is quite a lot of code to be added se we should better document
> > what it is intended for. Sure I understand your reasonaning about huge
> > pages are more likely to fail but is this really worth a separate
> > counter? Do you have an example of how this would be useful?
> 
> Our customers periodically collect some log info to understand what
> happened after system failures happen.  Then if we have separate counters
> for hugepage migration and the values show some anomaly, that might
> help admins and developers understand the issue more quickly.
> We have other ways to get this info like checking /proc/pid/pagemap and
> /proc/kpageflags, but they are costly and most users decide not to
> collect them in periodical logging.

Wouldn't tracepoints be more suitable for that purpose? They can collect
more valuable information.

> > If we are there then what about different huge page sizes (for hugetlb)?
> > Do we need per-hstate stats?
> 
> Yes, per-hstate counters are better. And existing hugetlb counters
> htlb_buddy_alloc_* are also affected by this point.

The thing is that this would bloat the code and the vmstat output even more.
I am not really convinced this is a great idea for something that
tracepoints would handle as well.
-- 
Michal Hocko
SUSE Labs