netdev - Re: [PATCH 0/4] net: mitigating kmem_cache slowpath for network stack in NAPI context

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Date:	Mon, 26 Oct 2015 18:09:39 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	brouer@...hat.com
Cc:	netdev@...r.kernel.org, alexander.duyck@...il.com,
	linux-mm@...ck.org, iamjoonsoo.kim@....com,
	akpm@...ux-foundation.org, cl@...ux.com
Subject: Re: [PATCH 0/4] net: mitigating kmem_cache slowpath for network
 stack in NAPI context

From: Jesper Dangaard Brouer <brouer@...hat.com>
Date: Fri, 23 Oct 2015 14:46:01 +0200

> It have been a long road. Back in July 2014 I realized that network
> stack were hitting the kmem_cache/SLUB slowpath when freeing SKBs, but
> had no solution.  In Dec 2014 I had implemented a solution called
> qmempool[1], that showed it was possible to improve this, but got
> rejected due to being a cache on top of kmem_cache.  In July 2015
> improvements to kmem_cache were proposed, and recently Oct 2015 my
> kmem_cache (SLAB+SLUB) patches for bulk alloc and free have been
> accepted into the AKPM quilt tree.
> 
> This patchset is the first real use-case kmem_cache bulk alloc and free.
> And is joint work with Alexander Duyck while still at Red Hat.
> 
> Using bulk free to avoid the SLUB slowpath shows the full potential.
> In this patchset it is realized in NAPI/softirq context.  1. During
> DMA TX completion bulk free is optimal and does not introduce any
> added latency. 2. bulk free of SKBs delay free'ed due to IRQ context
> in net_tx_action softirq completion queue.
> 
> Using bulk alloc is showing minor improvements for SLUB(+0.9%), but a
> very slight slowdown for SLAB(-0.1%).
> 
> [1] http://thread.gmane.org/gmane.linux.network/342347/focus=126138
> 
> 
> This patchset is based on net-next (commit 26440c835), BUT I've
> applied several patches from AKPMs MM-tree.
> 
> Cherrypick some commits from MMOTM tree on branch/tag mmotm-2015-10-06-16-30
> from git://git.kernel.org/pub/scm/linux/kernel/git/mhocko/mm.git
> (Below commit IDs are obviously not stable)

Logically I'm fine with this series, but as you mention there are
dependencies that need to hit upstream before I can merge any of
this stuff into my tree.

I also think that patch #4 is a net-win, and also will expose the
bulking code to more testing since it will be used more often.
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