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.
next prev parent 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 [PATCH] Add mmap_assert_locked() annotations to find_vma*() 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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