linux-kernel - Re: [PATCH] mm: Do not stall in synchronous compaction for THP allocations

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Message-ID: <CAEwNFnCRCxrru5rBk7FpypqeL8nD=SY5W3-TaA7Ap5o4CgDSbg@mail.gmail.com>
Date:	Fri, 11 Nov 2011 00:12:01 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Andy Isaacson <adi@...apodia.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: Do not stall in synchronous compaction for THP allocations

Hi Mel,

You should have Cced with me because __GFP_NORETRY is issued by me.

On Thu, Nov 10, 2011 at 11:22 PM, Mel Gorman <mgorman@...e.de> wrote:
> On Thu, Nov 10, 2011 at 10:06:16AM +0000, Mel Gorman wrote:
>> than stall. It was suggested that __GFP_NORETRY be used instead of
>> __GFP_NO_KSWAPD. This would look less like a special case but would
>> still cause compaction to run at least once with sync compaction.
>>
>
> This comment is bogus - __GFP_NORETRY would have caught THP allocations
> and would not call sync compaction. The issue was that it would also
> have caught any hypothetical high-order GFP_THISNODE allocations that
> end up calling compaction here

In fact, the I support patch concept so I would like to give

Acked-by: Minchan Kim <minchan.kim@...il.com>
But it is still doubt about code.

__GFP_NORETRY: The VM implementation must not retry indefinitely

What could people think if they look at above comment?
At least, I can imagine two

First, it is related on *latency*.
Second, "I can handle if VM fails allocation"

I am biased toward latter.
Then, __GFP_NO_KSWAPD is okay? It means "let's avoid sync compaction
or long latency"?
It's rather awkward name. Already someone started to use
__GFP_NO_KSWAPD as such purpose.
See mtd_kmalloc_up_to. He mentioned in comment of function as follows,

 * the system page size. This attempts to make sure it does not adversely
 * impact system performance, so when allocating more than one page, we
 * ask the memory allocator to avoid re-trying, swapping, writing back
 * or performing I/O.

That thing was what I concerned.
In future, new users of __GFP_NO_KSWAPD is coming and we can't prevent
them under our sight.
So I hope we can change the flag name or fix above code and comment
out __GFP_NO_KSWAPD

/*
 * __GFP_NO_KSWAPD is very VM internal flag so Please don't use it
without allowing mm guys
 *
#define __GFP_NO_KSWAPD xxxx

>
>                /*
>                 * High-order allocations do not necessarily loop after
>                 * direct reclaim and reclaim/compaction depends on
>                 * compaction being called after reclaim so call directly if
>                 * necessary
>                 */
>                page = __alloc_pages_direct_compact(gfp_mask, order,
>                                        zonelist, high_zoneidx,
>                                        nodemask,
>                                        alloc_flags, preferred_zone,
>                                        migratetype, &did_some_progress,
>                                        sync_migration);
>
> __GFP_NORETRY is used in a bunch of places and while the most
> of them are not high-order, some of them potentially are like in
> sound/core/memalloc.c. Using __GFP_NO_KSWAPD as the flag allows
> these callers to continue using sync compaction.  It could be argued

Okay. If I was biased first, I have opposed this comment because they
might think __GFP_NORETRY is very latency sensitive.
So they wanted allocation is very fast without any writeback/retrial.
In view point, __GFP_NORETRY isn't bad, I think.

Having said that, I was biased latter, as I said earlier.

> that they would prefer __GFP_NORETRY but the potential side-effects
> should be taken should be taken into account and the comment updated

Considering side-effect, your patch is okay.
But I can't understand you mentioned "the comment updated if that
happens" sentence. :(

> if that happens.
>
> --
> Mel Gorman
> SUSE Labs
>
> --
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-- 
Kind regards,
Minchan Kim
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