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Date:   Tue, 18 Dec 2018 15:07:45 +0000
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     He Zhe <zhe.he@windriver.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        tglx@linutronix.de, rostedt@goodmis.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH v2] kmemleak: Turn kmemleak_lock to raw spinlock on RT
Message-ID: <20181218150744.GB20197@arrakis.emea.arm.com>
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On Tue, Dec 18, 2018 at 06:51:41PM +0800, He Zhe wrote:
> On 2018/12/6 03:14, Sebastian Andrzej Siewior wrote:
> > With raw locks you wouldn't have multiple readers at the same time.
> > Maybe you wouldn't have recursion but since you can't have multiple
> > readers you would add lock contention where was none (because you could
> > have two readers at the same time).
> 
> OK. I understand your concern finally. At the commit log said, I wanted to use raw
> rwlock but didn't find the DEFINE helper for it. Thinking it would not be expected to
> have great performance, I turn to use raw spinlock instead. And yes, this would
> introduce worse performance.

Looking through the kmemleak code, I can't really find significant
reader contention. The longest holder of this lock (read) is the scan
thread which is also protected by a scan_mutex, so can't run
concurrently with another scanner (triggered through debugfs). The other
read_lock(&kmemleak_lock) user is find_and_get_object() called from a
few places. However, all such places normally follow a create_object()
call (kmemleak_alloc() and friends) which already performs a
write_lock(&kmemleak_lock), so it needs to wait for the scan thread to
release the kmemleak_lock.

It may be worth running some performance/latency tests during kmemleak
scanning (echo scan > /sys/kernel/debug/kmemleak) but at a quick look,
I don't think we'd see any difference with a raw_spin_lock_t.

With a bit more thinking (though I'll be off until the new year), we
could probably get rid of the kmemleak_lock entirely in scan_block() and
make lookup_object() and the related rbtree code in kmemleak RCU-safe.

-- 
Catalin