linux-kernel - Re: [PATCH V4] mm/vmstat: Add events for THP migration without split

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Date:   Thu, 9 Jul 2020 13:15:15 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Zi Yan <ziy@...dia.com>, Randy Dunlap <rdunlap@...radead.org>,
        "Anshuman Khandual" <anshuman.khandual@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, Jonathan Corbet <corbet@....net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4] mm/vmstat: Add events for THP migration without split

On 2020-07-09 10:42, Zi Yan wrote:
...
> diff --git a/Documentation/vm/page_migration.rst b/Documentation/vm/page_migration.rst
> index e65d49f3cf86..68883ac485fa 100644
> --- a/Documentation/vm/page_migration.rst
> +++ b/Documentation/vm/page_migration.rst
> @@ -253,24 +253,32 @@ which are function pointers of struct address_space_operations.
>        PG_isolated is alias with PG_reclaim flag so driver shouldn't use the flag
>        for own purpose.
> 
> -Quantifying Migration
> +Monitoring Migration
>   =====================
> -Following events can be used to quantify page migration.
> -
> -1. PGMIGRATE_SUCCESS       /* Normal page migration success */
> -2. PGMIGRATE_FAIL          /* Normal page migration failure */
> -3. THP_MIGRATION_SUCCESS   /* Transparent huge page migration success */
> -4. THP_MIGRATION_FAILURE   /* Transparent huge page migration failure */
> -5. THP_MIGRATION_SPLIT     /* Transparent huge page got split, retried */
> -
> -THP_MIGRATION_SUCCESS is when THP is migrated successfully without getting
> -split into it's subpages. THP_MIGRATION_FAILURE is when THP could neither
> -be migrated nor be split. THP_MIGRATION_SPLIT is when THP could not
> -just be migrated as is but instead get split into it's subpages and later
> -retried as normal pages. THP events would also update normal page migration
> -statistics PGMIGRATE_SUCCESS and PGMIGRATE_FAILURE. These events will help
> -in quantifying and analyzing various THP migration events including both
> -success and failure cases.
> +
> +The following events (counters) can be used to monitor page migration.
> +
> +1. PGMIGRATE_SUCCESS: Normal page migration success. Each count means that a
> +   page was migrated. If the page was a non-THP page, then this counter is
> +   increased by one. If the page was a THP, then this counter is increased by
> +   the number of THP subpages. For example, migration of a single 2MB THP that
> +   has 4KB-size base pages (subpages) will cause this counter to increase by
> +   512.
> +
> +2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for
> +   _SUCCESS, above: this will be increased by the number of subpages, if it was
> +   a THP.
> +
> +3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.
> +
> +4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split.
> +
> +5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had
> +   to be split. After splitting, a migration retry was used for it's sub-pages.


Just a documentation nit: "its sub-pages", not "it's sub-pages".


thanks,
-- 
John Hubbard
NVIDIA