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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Christoph Lameter <cl@linux.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH] mm: fix draining remote pageset
References: <20230811090819.60845-1-ying.huang@intel.com>
        <ZNYA6YWLqtDOdQne@dhcp22.suse.cz>
Date:   Mon, 14 Aug 2023 09:59:51 +0800
In-Reply-To: <ZNYA6YWLqtDOdQne@dhcp22.suse.cz> (Michal Hocko's message of
        "Fri, 11 Aug 2023 11:35:37 +0200")
Message-ID: <87r0o6bcyw.fsf@yhuang6-desk2.ccr.corp.intel.com>
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Hi, Michal,

Michal Hocko <mhocko@suse.com> writes:

> On Fri 11-08-23 17:08:19, Huang Ying wrote:
>> If there is no memory allocation/freeing in the remote pageset after
>> some time (3 seconds for now), the remote pageset will be drained to
>> avoid memory wastage.
>> 
>> But in the current implementation, vmstat updater worker may not be
>> re-queued when we are waiting for the timeout (pcp->expire != 0) if
>> there are no vmstat changes, for example, when CPU goes idle.
>
> Why is that a problem?

The pages of the remote zone may be kept in the local per-CPU pageset
for long time as long as there's no page allocation/freeing on the
logical CPU.  In addition to the logical CPU goes idle, this is also
possible if the logical CPU is busy in the user space.

I will update the change log to include this.

--
Best Regards,
Huang, Ying

>> This is fixed via guaranteeing that the vmstat updater worker will
>> always be re-queued when we are waiting for the timeout.
>> 
>> We can reproduce the bug via allocating/freeing pages from remote
>> node, then go idle.  And the patch can fix it.
>> 
>> Fixes: 7cc36bbddde5 ("vmstat: on-demand vmstat workers V8")
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> ---
>>  mm/vmstat.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>> index b731d57996c5..111118741abf 100644
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -856,8 +856,10 @@ static int refresh_cpu_vm_stats(bool do_pagesets)
>>  				continue;
>>  			}
>>  
>> -			if (__this_cpu_dec_return(pcp->expire))
>> +			if (__this_cpu_dec_return(pcp->expire)) {
>> +				changes++;
>>  				continue;
>> +			}
>>  
>>  			if (__this_cpu_read(pcp->count)) {
>>  				drain_zone_pages(zone, this_cpu_ptr(pcp));
>> -- 
>> 2.39.2