From: Mel Gorman <mgorman@suse.de>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm: fix pgalloc_stall on unpopulated zone
Date: Thu, 14 Jul 2016 09:51:53 +0100 [thread overview]
Message-ID: <20160714085153.GL11400@suse.de> (raw)
In-Reply-To: <20160714011119.GA23512@bbox>
On Thu, Jul 14, 2016 at 10:11:19AM +0900, Minchan Kim wrote:
> > The patch means that the vmstat accounting and tracepoint data is also
> > out of sync. One thing I wanted to be able to do was
> >
> > 1. Observe that there are alloc stalls on DMA32 or some other low zone
> > 2. Activate mm_vmscan_direct_reclaim_begin, filter on classzone_idx ==
> > DMA32 and identify the source of the lowmem allocations
> >
> > If your patch is applied, I cannot depend on the stall stats any more
> > and the tracepoint is required to determine if there really any
> > zone-contrained allocations. It can be *inferred* from the skip stats
> > but only if such skips occurred and that is not guaranteed.
>
> Just a nit:
>
> Hmm, can't we omit classzone_idx in mm_vm_scan_direct_begin_template?
> Because every functions already have gfp_flags so that we can classzone_idx
> via gfp_zone(gfp_flags) without passing it.
>
We could but it's potentially wrong. classzone_idx *should* be derived
from the gfp_flags but it's possible a bug would lead it to be another
value. The saving from passing it in is marginal at best.
If it's omitted from the tracepoint itself, there is a small amount of
disk saving which is potentially significant if there is a lot of direct
reclaim. Unfortunately, it also makes it harder to filter that
tracepoint because the filter rules must be an implementation of
gfp_zone.
Right now I believe the saving is marginal and the cost of potentially
using the wrong information or making the filtering harder offsets that
marginal saving.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2016-07-14 8:52 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-13 2:24 [PATCH] mm: fix pgalloc_stall on unpopulated zone Minchan Kim
2016-07-13 9:25 ` Mel Gorman
2016-07-14 1:11 ` Minchan Kim
2016-07-14 8:51 ` Mel Gorman [this message]
2016-07-14 8:58 ` Minchan Kim
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