linux-kernel - Re: [PATCH V3] mm/vmstat: Add events for THP migration without split

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Date:   Tue, 7 Jul 2020 14:57:18 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>
CC:     <linux-mm@...ck.org>, Daniel Jordan <daniel.m.jordan@...cle.com>,
        "Hugh Dickins" <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>, Zi Yan <ziy@...dia.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3] mm/vmstat: Add events for THP migration without split

On 2020-07-07 13:04, Andrew Morton wrote:
> On Tue,  7 Jul 2020 05:36:55 +0530 Anshuman Khandual <anshuman.khandual@....com> wrote:
> 
>> Add following new vmstat events which will help in validating THP migration
>> without split. Statistics reported through these new VM events will help in
>> performance debugging.
>>
>> 1. THP_MIGRATION_SUCCESS
>> 2. THP_MIGRATION_FAILURE
>> 3. THP_MIGRATION_SPLIT
>>
>> In addition, these new events also update normal page migration statistics
>> appropriately via PGMIGRATE_SUCCESS and PGMIGRATE_FAILURE. While here, this
>> updates current trace event 'mm_migrate_pages' to accommodate now available
>> THP statistics.
> 
> Patch looks straightforward enough.  It would be nice to see some
> confirmation from others that these metrics are a desirable thing to
> export.

Taking a peek now.

> 
>> ...
>>
>> -	trace_mm_migrate_pages(nr_succeeded, nr_failed, mode, reason);
>> +	if (nr_thp_succeeded)
>> +		count_vm_events(THP_MIGRATION_SUCCESS, nr_thp_succeeded);
>> +	if (nr_thp_failed)
>> +		count_vm_events(THP_MIGRATION_FAILURE, nr_thp_failed);
>> +	if (nr_thp_split)
>> +		count_vm_events(THP_MIGRATION_SPLIT, nr_thp_split);
> 
> Are these "if"s worthwhile to have?

No, they are not. And there are a couple more pre-existing cases as well,
right above what can be seen here (this patch just follows the local pattern)
that should also be removed.



thanks,
-- 
John Hubbard
NVIDIA