linux-kernel - Re: [PATCH] mm/vmstat: Add events for PMD based THP migration without split

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Date:   Mon, 1 Jun 2020 20:20:05 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Anshuman Khandual <anshuman.khandual@....com>
CC:     <linux-mm@...ck.org>, <hughd@...gle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Zi Yan <ziy@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/vmstat: Add events for PMD based THP migration without
 split

On 2020-06-01 09:57, Daniel Jordan wrote:
> Hi Anshuman,
> 
> On Fri, May 22, 2020 at 09:04:04AM +0530, Anshuman Khandual wrote:
>> This adds the following two new VM events which will help in validating PMD
>> based THP migration without split. Statistics reported through these events
>> will help in performance debugging.
>>
>> 1. THP_PMD_MIGRATION_SUCCESS
>> 2. THP_PMD_MIGRATION_FAILURE
> 
> The names suggest a binary event similar to the existing
> pgmigrate_success/fail, but FAILURE only tracks one kind of migration error,
> and then only when the thp is successfully split, so shouldn't it be called
> SPLIT instead?
> 

So the full description of the situation, which we're trying to compress into
a shorter name, is "THP pmd migration failure, due to successfully splitting
the THP". From that, the beginning part is the real point here, while the last
part is less important. In other words, the users of these events are people
who are trying to quantify THP migrations, and these events are particularly
relevant for that. The "THP migration failed" is more important here than
the reason that it failed. Or so I believe so far.

So I still think that the names are really quite good, but your point is
also important: maybe this patch should also be tracking other causes
of THP PMD migration failure, in order to get a truer accounting of the
situation.

thanks,
-- 
John Hubbard
NVIDIA