Re: [PATCH v6] mm: Add memory allocation watchdog kernel thread.

[Tetsuo Handa]

Michal Hocko wrote at http://lkml.kernel.org/r/20161227105715.GE1308@xxxxxxxxxxxxxx :
> On Tue 27-12-16 19:39:28, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > I am not saying that the current code works perfectly when we are
> > > hitting the direct reclaim close to the OOM but improving that requires
> > > much more than slapping a global lock there.
> > 
> > So, we finally agreed that there are problems when we are hitting the direct
> > reclaim close to the OOM. Good.
> 
> There has never been a disagreement here. The point we seem to be
> disagreeing is how much those issues you are seeing matter. I do not
> consider them top priority because they are not happening in real life
> enough.

There is no evidence to prove "they are not happening in real life enough", for
there is no catch-all reporting mechanism. I consider that offering a mean to
find and report problems is top priority as a troubleshooting staff.

> > > Just try to remember how you were pushing really hard for oom timeouts
> > > one year back because the OOM killer was suboptimal and could lockup. It
> > > took some redesign and many changes to fix that. The result is
> > > imho a better, more predictable and robust code which wouldn't be the
> > > case if we just went your way to have a fix quickly...
> > 
> > I agree that the result is good for users who can update kernels. But that
> > change was too large to backport. Any approach which did not in time for
> > customers' deadline of deciding their kernels to use for 10 years is
> > useless for them. Lack of catch-all reporting/triggering mechanism is
> > unhappy for both customers and troubleshooting staffs at support centers.
> 
> Then implement whatever you find appropriate on those old kernels and
> deal with the follow up reports. This is the fair deal you have cope
> with when using and supporting old kernels.

Customers are using distributor's kernels. Due to out-of-tree vendor's prebuilt
modules which can be loaded into only prebuilt distributor's kernels, it is
impossible for me to make changes to those old kernels. Also, that distributor's
policy is that "offer no support even if just rebuilt from source" which prevents
customers from testing changes made by me to those old kernels. Thus, implement
whatever I find appropriate on those old kernels is not an option. Merging
upstream-first, in accordance with that distributor's policy, is the only option.

>  
> > Improving the direct reclaim close to the OOM requires a lot of effort.
> > We might add new bugs during that effort. So, where is valid reason that
> > we can not have asynchronous watchdog like kmallocwd? Please do explain
> > at kmallocwd thread. You have never persuaded me about keeping kmallocwd
> > out of tree.
> 
> I am not going to repeat my arguments over again. I haven't nacked that
> patch and it seems there is no great interest in it so do not try to
> claim that it is me who is blocking this feature. I just do not think it
> is worth it.

OK. I was assuming that Acked-by: or Reviewed-by: from you is essential.

So far, nobody has objections about having asynchronous watchdog.
Mel, Johannes and Vladimir, what do you think about this version of
kmallocwd? If no objections, I think we can start with this version
with a fix shown below folded.

----------------------------------------
>From 5adc8d9bfb31dce1954667cabf65842df31d4ed7 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Date: Wed, 28 Dec 2016 09:52:03 +0900
Subject: [PATCH] mm: Don't check __GFP_KSWAPD_RECLAIM by memory allocation
 watchdog.

There are some __GFP_KSWAPD_RECLAIM && !__GFP_DIRECT_RECLAIM callers.
Since such callers do not sleep, we should check only __GFP_DIRECT_RECLAIM
callers than __GFP_RECLAIM == (__GFP_KSWAPD_RECLAIM|__GFP_DIRECT_RECLAIM)
callers.

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
---
 mm/page_alloc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6478f44..58c1238 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3769,10 +3769,10 @@ static void start_memalloc_timer(const gfp_t gfp_mask, const int order)
 {
 	struct memalloc_info *m = &current->memalloc;
 
-	/* We don't check for stalls for !__GFP_RECLAIM allocations. */
-	if (!(gfp_mask & __GFP_RECLAIM))
+	/* We don't check for stalls for !__GFP_DIRECT_RECLAIM allocations. */
+	if (!(gfp_mask & __GFP_DIRECT_RECLAIM))
 		return;
-	/* We don't check for stalls for nested __GFP_RECLAIM allocations */
+	/* Check based on outermost __GFP_DIRECT_RECLAIM allocations. */
 	if (!m->valid) {
 		m->sequence++;
 		m->start = jiffies;
@@ -3788,7 +3788,7 @@ static void stop_memalloc_timer(const gfp_t gfp_mask)
 {
 	struct memalloc_info *m = &current->memalloc;
 
-	if ((gfp_mask & __GFP_RECLAIM) && !--m->valid)
+	if ((gfp_mask & __GFP_DIRECT_RECLAIM) && !--m->valid)
 		this_cpu_dec(memalloc_in_flight[m->idx]);
 }
 #else
-- 
1.8.3.1