From: Michal Hocko <mhocko@suse.cz>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Eric Dumazet <edumazet@google.com>,
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 17:47:16 +0200 [thread overview]
Message-ID: <20150618154716.GH5858@dhcp22.suse.cz> (raw)
In-Reply-To: <5582E240.8080704@suse.cz>
On Thu 18-06-15 17:22:40, Vlastimil Babka wrote:
> 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?)
That is to be discussed. Most allocations already express their interest
in memory reserves by __GFP_HIGH directly or by GFP_ATOMIC indirectly.
So maybe we do not need any additional flag here. There are not that
many ~__GFP_WAIT and most of them seem to require it _only_ because the
context doesn't allow for sleeping (e.g. to prevent from deadlocks).
> 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.
I agree. So I think that maybe we should drop that implicit access to
memory reserves for ~__GFP_WAIT allocations and let it do what it is
documented to do.
> 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...
I think simply dropping GFP_WAIT is a good way to express that. The
fact that the current implementation gives access to memory reserves
implicitly is just a detail and the user of the allocator shouldn't care
about that.
--
Michal Hocko
SUSE Labs
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WARNING: multiple messages have this Message-ID (diff)
From: Michal Hocko <mhocko@suse.cz>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Eric Dumazet <edumazet@google.com>,
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 17:47:16 +0200 [thread overview]
Message-ID: <20150618154716.GH5858@dhcp22.suse.cz> (raw)
In-Reply-To: <5582E240.8080704@suse.cz>
On Thu 18-06-15 17:22:40, Vlastimil Babka wrote:
> 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?)
That is to be discussed. Most allocations already express their interest
in memory reserves by __GFP_HIGH directly or by GFP_ATOMIC indirectly.
So maybe we do not need any additional flag here. There are not that
many ~__GFP_WAIT and most of them seem to require it _only_ because the
context doesn't allow for sleeping (e.g. to prevent from deadlocks).
> 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.
I agree. So I think that maybe we should drop that implicit access to
memory reserves for ~__GFP_WAIT allocations and let it do what it is
documented to do.
> 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...
I think simply dropping GFP_WAIT is a good way to express that. The
fact that the current implementation gives access to memory reserves
implicitly is just a detail and the user of the allocator shouldn't care
about that.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2015-06-18 15:47 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
2015-06-18 15:47 ` Michal Hocko [this message]
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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