From: Vlastimil Babka <vbabka@suse.cz>
To: Michal Hocko <mhocko@suse.cz>, Eric Dumazet <edumazet@google.com>
Cc: David Rientjes <rientjes@google.com>, Shaohua Li <shli@fb.com>,
netdev <netdev@vger.kernel.org>,
David Miller <davem@davemloft.net>,
kernel-team <Kernel-team@fb.com>,
clm@fb.com, linux-mm@kvack.org, dbavatar@gmail.com
Subject: Re: [RFC V3] net: don't wait for order-3 page allocation
Date: Thu, 18 Jun 2015 17:22:40 +0200 [thread overview]
Message-ID: <5582E240.8080704@suse.cz> (raw)
In-Reply-To: <20150618144311.GF5858@dhcp22.suse.cz>
On 06/18/2015 04:43 PM, Michal Hocko wrote:
> On Thu 18-06-15 07:35:53, Eric Dumazet wrote:
>> On Thu, Jun 18, 2015 at 7:30 AM, Michal Hocko <mhocko@suse.cz> wrote:
>>
>>> Abusing __GFP_NO_KSWAPD is a wrong way to go IMHO. It is true that the
>>> _current_ implementation of the allocator has this nasty and very subtle
>>> side effect but that doesn't mean it should be abused outside of the mm
>>> proper. Why shouldn't this path wake the kswapd and let it compact
>>> memory on the background to increase the success rate for the later
>>> high order allocations?
>>
>> I kind of agree.
>>
>> If kswapd is a problem (is it ???) we should fix it, instead of adding
>> yet another flag to some random locations attempting
>> memory allocations.
>
> No, kswapd is not a problem. The problem is ~__GFP_WAIT allocation can
> access some portion of the memory reserves (see gfp_to_alloc_flags resp.
> __zone_watermark_ok and ALLOC_HARDER). __GFP_NO_KSWAPD is just a dirty
> hack to not give that access which was introduced for THP AFAIR.
>
> The implicit access to memory reserves for non sleeping allocation has
> been there for ages and it might be not suitable for this particular
> path but that doesn't mean another gfp flag with a different side effect
> should be hijacked. We should either stop doing that implicit access to
> memory reserves and give __GFP_RESERVE or add the __GFP_NORESERVE. But
> that is a problem to be solved in the mm proper. Spreading subtle
> dependencies outside of mm will just make situation worse.
So you are not proposing to use these __GFP_RESERVE/NORESERVE flag
outside of mm, right? (besides, we distinguish several kinds of
reserves, so what exactly would the flag do?) As that would be also
subtle dependency. The general problem I think is that we should want
the mm users to specify higher-level intentions (such as GFP_KERNEL)
which would map to specific directions (__GFP_*) for the allocator, and
currently it's rather a mess of both kinds of flags. Clearly the
intention here is "opportunistic allocation that should not
reclaim/compact, use reserves, wake up kswapd (?) because it's better to
fall back to smaller pages than wait") and we don't seem to have a
GFP_OPPORTUNISTIC flag for that. The allocation has to then mask out
__GFP_WAIT which however looks like an atomic allocation to the
allocator and give access to reserves, etc...
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next prev parent reply other threads:[~2015-06-18 15:22 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-11 23:50 [RFC V3] net: don't wait for order-3 page allocation Shaohua Li
2015-06-11 23:50 ` Shaohua Li
2015-06-12 0:02 ` Eric Dumazet
2015-06-12 0:02 ` Eric Dumazet
2015-06-12 0:34 ` David Miller
2015-06-12 0:34 ` David Miller
2015-06-12 9:36 ` Vlastimil Babka
2015-06-12 9:36 ` Vlastimil Babka
2015-06-17 23:02 ` David Rientjes
2015-06-17 23:02 ` David Rientjes
2015-06-18 14:30 ` Michal Hocko
2015-06-18 14:30 ` Michal Hocko
2015-06-18 14:35 ` Eric Dumazet
2015-06-18 14:43 ` Michal Hocko
2015-06-18 14:43 ` Michal Hocko
2015-06-18 15:22 ` Vlastimil Babka [this message]
2015-06-18 15:47 ` Michal Hocko
2015-06-18 15:47 ` Michal Hocko
2015-06-30 23:49 ` David Rientjes
2015-06-30 23:49 ` David Rientjes
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