linux-kernel - Re: [RFC PATCH] posix timers: allocate timer id per task

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Message-ID: <1350318883.3954.321.camel@edumazet-glaptop>
Date:	Mon, 15 Oct 2012 18:34:43 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	peterz@...radead.org, mingo@...hat.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, devel@...nvz.org
Subject: Re: [RFC PATCH] posix timers: allocate timer id per task

On Mon, 2012-10-15 at 20:17 +0400, Stanislav Kinsbursky wrote:
> This patch is required CRIU project (www.criu.org).
> To migrate processes with posix timers we have to make sure, that we can
> restore posix timer with proper id.
> Currently, this is not true, because timer ids are allocated globally.
> So, this is precursor patch and it's purpose is make posix timer id to be
> allocated per task.
> 
> Patch replaces global idr with global hash table for posix timers and
> makes timer ids unique not globally, but per task. Next free timer id is type
> of integer and stored on signal struct (posix_timer_id). If free timer id
> reaches negative value on timer creation, it will be dropped to zero and
> -EAGAIN will be returned to user.
> Hash table is size of page (4KB).
> Key is constructed as follows:
> key = hash_ptr(current->signal) ^ hash_32(posix_timer_id);
> 
> Signed-off-by: Stanislav Kinsbursky <skinsbursky@...allels.com>


Hmm, it seems you removed idr, rcu friendly, and reinstated a fixed size
hash table, protected by a _single_ spinlock ? Oh well.

Please take a look at commit 8af088710d1e, and make sure you fix your
problem and keep performance as good as before.



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