From: Charan Teja Kalla <charante@codeaurora.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
vinmenon@codeaurora.org
Subject: Re: [PATCH] mm, page_alloc: reset the zone->watermark_boost early
Date: Tue, 12 May 2020 19:01:22 +0530 [thread overview]
Message-ID: <1cf5e778-eae1-fc71-aed4-d84d664d79dd@codeaurora.org> (raw)
In-Reply-To: <20200511131155.0b40ee443c3367e8f748b16f@linux-foundation.org>
Thank you Andrew for the reply.
On 5/12/2020 1:41 AM, Andrew Morton wrote:
> On Mon, 11 May 2020 19:10:08 +0530 Charan Teja Reddy <charante@codeaurora.org> wrote:
>
>> Updating the zone watermarks by any means, like extra_free_kbytes,
>> min_free_kbytes, water_mark_scale_factor e.t.c, when watermark_boost is
>> set will result into the higher low and high watermarks than the user
>> asks. This can be avoided by resetting the zone->watermark_boost to zero
>> early.
>
> Does this solve some problem which has been observed in testing?
Sorry, what are those issues observed in testing? It would be helpful
If you post them here.
>
>> ...
>>
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7746,9 +7746,9 @@ static void __setup_per_zone_wmarks(void)
>> mult_frac(zone_managed_pages(zone),
>> watermark_scale_factor, 10000));
>>
>> + zone->watermark_boost = 0;
>> zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp;
>> zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
>> - zone->watermark_boost = 0;
>>
>> spin_unlock_irqrestore(&zone->lock, flags);
>> }
>
> This could only be a problem if code is accessing these things without
> holding zone->lock. Is that ever the case?
>
This is a problem even when accessing these things with zone->lock
held because we are directly using the macro min_wmark_pages(zone)
which leads to the issue. Pasting macro here for reference.
#define min_wmark_pages(z) (z->_watermark[WMARK_MIN] + z->watermark_boost)
Steps that lead to the issue is like below:
1) On the extfrag event, we try to boost the watermark by storing the
value in ->watermark_boost.
2) User changes the value of extra|min_free_kbytes or watermark_scale_factor.
In __setup_perzone_wmarks, we directly store the user asked
watermarks in the zones structure. In this step, the value
is always offsets by ->watermark_boost as we use the min_wmark_pages() macro.
3) Later, when kswapd woke up, it resets the zone's watermark_boost to zero.
Step 2 from the above is what resulting into the issue.
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
next prev parent reply other threads:[~2020-05-12 13:32 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-11 13:40 [PATCH] mm, page_alloc: reset the zone->watermark_boost early Charan Teja Reddy
2020-05-11 20:11 ` Andrew Morton
2020-05-12 13:31 ` Charan Teja Kalla [this message]
2020-05-13 9:46 ` Charan Teja Kalla
2020-05-13 22:24 ` Andrew Morton
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