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From:   Zi Yan <ziy@nvidia.com>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
CC:     <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [RFC] mm/vmstat: Add events for THP migration without split
Date:   Thu, 14 May 2020 10:28:53 -0400
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On 12 May 2020, at 0:22, Anshuman Khandual wrote:

> Add the following new trace events which will help in validating migrat=
ion
> events involving PMD based THP pages.
>
> 1. THP_PMD_MIGRATION_ENTRY_SET
> 2. THP_PMD_MIGRATION_ENTRY_REMOVE
>
> There are no clear method to confirm whether a THP migration happened w=
ith
> out involving it's split. These trace events along with PGMIGRATE_SUCCE=
SS
> and PGMIGRATE_FAILURE will provide additional insights. After this chan=
ge,
>
> A single 2M THP (2K base page) when migrated
>
> 1. Without split
>
> ................
> pgmigrate_success 1
> pgmigrate_fail 0
> ................
> thp_pmd_migration_entry_set 1
> thp_pmd_migration_entry_remove 1
> ................
>
> 2. With split
>
> ................
> pgmigrate_success 512
> pgmigrate_fail 0
> ................
> thp_pmd_migration_entry_set 0
> thp_pmd_migration_entry_remove 0
> ................
>
> pgmigrate_success as 1 instead of 512, provides a hint for possible THP=

> migration event. But then it gets mixed with normal page migrations ove=
r
> time. These additional trace events provide required co-relation.

To track successful THP migrations, the code below should work, right?

diff --git a/mm/migrate.c b/mm/migrate.c
index b1092876e537..d394f5331288 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1220,6 +1220,8 @@ static ICE_noinline int unmap_and_move(new_page_t g=
et_new_page,
         * we want to retry.
         */
        if (rc =3D=3D MIGRATEPAGE_SUCCESS) {
+               if (PageTransHuge(newpage))
+                       count_vm_event(THP_PMD_MIGRATION_SUCCESS);
                put_page(page);
                if (reason =3D=3D MR_MEMORY_FAILURE) {
                        /*

Maybe you could give more details on why you want to add the THP migratio=
n event and
how you are going to use the event in your use case. That would be very h=
elpful to this
code review. Are you going to do anything if you see THP migration failur=
es?

Thanks.

=E2=80=94
Best Regards,
Yan Zi

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