kswapd endless loop for compaction

kswapd endless loop for compaction

[Johannes Weiner]

Hi guys,

while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into
a busy spin state without doing reclaim.  printk-style debugging told
me that this happens when the distance between a zone's high watermark
and its low watermark is less than two huge pages (DMA zone).

1. The first loop in balance_pgdat() over the zones finds all zones to
be above their high watermark and only does goto out (all_zones_ok).

2. pgdat_balanced() at the out: label also just checks the high
watermark, so the node is considered balanced and the order is not
reduced.

3. In the `if (order)' block after it, compaction_suitable() checks if
the zone's low watermark + twice the huge page size is okay, which
it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it
it go back to loop_again:.

This will go on until somebody else allocates and breaches the high
watermark and then hopefully goes on to reclaim the zone above low
watermark + 2 * THP.

I'm not really sure what the correct solution is.  Should we modify
the zone_watermark_ok() checks in balance_pgdat() to take into account
the higher watermark requirements for reclaim on behalf of compaction?
Change the check in compaction_suitable() / not use it directly?

Thanks,
Johannes

Re: kswapd endless loop for compaction

[Johannes Weiner]

Just to be clear, this is not fixed by Dave's patch to NR_FREE_PAGES
accounting.

I can still get 3.7-rc5 + Dave's fix to drop into an endless loop in
kswapd within a couple of minutes on my test box.

As described below, the bug comes from contradicting conditions in
balance_pgdat(), not an accounting problem.

On Tue, Nov 20, 2012 at 02:04:41PM -0500, Johannes Weiner wrote:
> Hi guys,
> 
> while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into
> a busy spin state without doing reclaim.  printk-style debugging told
> me that this happens when the distance between a zone's high watermark
> and its low watermark is less than two huge pages (DMA zone).
> 
> 1. The first loop in balance_pgdat() over the zones finds all zones to
> be above their high watermark and only does goto out (all_zones_ok).
> 
> 2. pgdat_balanced() at the out: label also just checks the high
> watermark, so the node is considered balanced and the order is not
> reduced.
> 
> 3. In the `if (order)' block after it, compaction_suitable() checks if
> the zone's low watermark + twice the huge page size is okay, which
> it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it
> it go back to loop_again:.
> 
> This will go on until somebody else allocates and breaches the high
> watermark and then hopefully goes on to reclaim the zone above low
> watermark + 2 * THP.
> 
> I'm not really sure what the correct solution is.  Should we modify
> the zone_watermark_ok() checks in balance_pgdat() to take into account
> the higher watermark requirements for reclaim on behalf of compaction?
> Change the check in compaction_suitable() / not use it directly?
> 
> Thanks,
> Johannes
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
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Re: kswapd endless loop for compaction

[Jaegeuk Hanse]

On 11/21/2012 03:04 AM, Johannes Weiner wrote:
> Hi guys,
>
> while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into
> a busy spin state without doing reclaim.  printk-style debugging told
> me that this happens when the distance between a zone's high watermark
> and its low watermark is less than two huge pages (DMA zone).
>
> 1. The first loop in balance_pgdat() over the zones finds all zones to
> be above their high watermark and only does goto out (all_zones_ok).
>
> 2. pgdat_balanced() at the out: label also just checks the high
> watermark, so the node is considered balanced and the order is not
> reduced.
>
> 3. In the `if (order)' block after it, compaction_suitable() checks if
> the zone's low watermark + twice the huge page size is okay, which
> it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it
> it go back to loop_again:.
>
> This will go on until somebody else allocates and breaches the high
> watermark and then hopefully goes on to reclaim the zone above low
> watermark + 2 * THP.
>
> I'm not really sure what the correct solution is.  Should we modify
> the zone_watermark_ok() checks in balance_pgdat() to take into account
> the higher watermark requirements for reclaim on behalf of compaction?
> Change the check in compaction_suitable() / not use it directly?
>

Hi Johannes,


- If all zones meet high watermark, goto out, then why go to `if 
(order)' block?

- If depend on compaction get enough contigous pages, why


if (CONPACT_BUILD && order &&

     compaction_suitable(zone, order) !=

         COMPACTION_SKIPPED)

     testorder = 0;


can't guarantee low watermark + twice the huge page size is okay?


Regards,

Jaegeuk

>
> Thanks,
> Johannes
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

Re: kswapd endless loop for compaction

[Jaegeuk Hanse]

On 11/21/2012 03:04 AM, Johannes Weiner wrote:
> Hi guys,
>
> while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into
> a busy spin state without doing reclaim.  printk-style debugging told
> me that this happens when the distance between a zone's high watermark
> and its low watermark is less than two huge pages (DMA zone).
>
> 1. The first loop in balance_pgdat() over the zones finds all zones to
> be above their high watermark and only does goto out (all_zones_ok).
>
> 2. pgdat_balanced() at the out: label also just checks the high
> watermark, so the node is considered balanced and the order is not
> reduced.
>
> 3. In the `if (order)' block after it, compaction_suitable() checks if
> the zone's low watermark + twice the huge page size is okay, which
> it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it
> it go back to loop_again:.
>
> This will go on until somebody else allocates and breaches the high
> watermark and then hopefully goes on to reclaim the zone above low
> watermark + 2 * THP.
>
> I'm not really sure what the correct solution is.  Should we modify
> the zone_watermark_ok() checks in balance_pgdat() to take into account
> the higher watermark requirements for reclaim on behalf of compaction?
> Change the check in compaction_suitable() / not use it directly?

Hi Johannes,

If depend on compaction get enough contigous pages, why

if (CONPACT_BUILD && order &&
     compaction_suitable(zone, order) !=
         COMPACTION_SKIPPED)
     testorder = 0;

can't guarantee low watermark + twice the huge page size is okay?

Regards,
Jaegeuk

> Thanks,
> Johannes
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>