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From: Andrew Morton <akpm@linux-foundation.org>
To: Luigi Rizzo <lrizzo@google.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	David Rientjes <rientjes@google.com>,
	linux-mm@kvack.org
Subject: Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*()
Date: Mon, 2 Aug 2021 14:11:14 -0700	[thread overview]
Message-ID: <20210802141114.7fe599b17d87cecbc4d5b70b@linux-foundation.org> (raw)
In-Reply-To: <CAMOZA0+mAC-tHDehzqMVP4rd7wggY_DPofRdH=GMouZA9DRC1w@mail.gmail.com>

On Mon, 2 Aug 2021 02:16:14 +0200 Luigi Rizzo <lrizzo@google.com> wrote:

> > Well, it isn't cost-free.  find_vma() is called a lot and a surprising
> > number of systems apparently run with CONFIG_DEBUG_VM.  Why do you
> > think this cost is justified?
> 
> I assume you are concerned with the cost of mmap_assert_locked() ?
> 
> I'd say the justification is the same as for all asserts:
> at some point some code change may miss the required lock, and the
> asserts are there to catch elusive race conditions,
> 
> There are in fact already instances of mmap_locked_assert()
> right before find_vma() in walk_page_range(), and a couple before
> calls to __get_user_pages().
> 
> As for the cost, I'd think that if CONFIG_DEBUG_VM is set,
> one does it on purpose to catch errors and is prepared to pay
> the cost (in this case the atomic_read(counter) in rwsem_is_locked(),
> the counter should be hot).
> 
> FWIW I have instrumented find_vma() on a fast machine using kstats
> 
>    https://github.com/luigirizzo/lr-cstats
> 
> (load the module then enable the trace with
>   echo "trace pcpu:find_vma bits 3" > /sys/kernel/debug/kstats/_control
> and monitor the time with
>   watch "grep CPUS /sys/kernel/debug/kstats/find_vma"
> 
> I didn't run anything especially intensive except some network
> benchmarks, but I have collected ~2M samples with the following
> distribution of find_vma() time in nanoseconds in 3 configs:
> 
> CONFIGURATION         p10   p50   p90   p95   p98
> 
> no-debug               89   109   214   332    605
> debug                 331   369   603   862   1338
> debug+this patch      337   369   603   863   1339
> 
> As you can see, just compiling a debug kernel, even without this patch,
> makes the function 3x more expensive. The effect of this patch is
> not measurable (the differences are below measurement error).

Cool, thanks, that's convincing.

  reply	other threads:[~2021-08-02 21:11 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-31 17:53 Luigi Rizzo
2021-08-01 19:33 ` Andrew Morton
2021-08-02  0:16   ` Luigi Rizzo
2021-08-02 21:11     ` Andrew Morton [this message]
2021-08-03 16:08 ` Jason Gunthorpe
2021-08-03 21:48   ` Luigi Rizzo
2021-08-03 23:07     ` Liam Howlett
2021-08-03 23:35       ` Jason Gunthorpe
2021-08-03 23:57         ` Luigi Rizzo
2021-08-04  5:12           ` Liam Howlett
2021-08-04 14:42       ` Jann Horn
2021-08-04 15:21         ` Jason Gunthorpe
2021-08-04 21:22           ` Jann Horn
2021-08-04 17:32         ` Liam Howlett

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