linux-kernel - Re: [PATCH 1/2] posix-timers: move global timer id management to signal

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Date:	Tue, 20 Sep 2011 00:23:27 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/2] posix-timers: move global timer id management to
 signal_struct v4

Le lundi 19 septembre 2011 à 14:48 -0700, Andi Kleen a écrit :
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Move the global posix timer ids IDR to signal_struct. This removes
> a minor global scalability bottleneck and also allows to finally limit
> the number of process timers in a sane way (see next patch)
> 
> I put it into signal_struct following the other posix timer per process
> structures.
> 
> v2: Now with locking again (thanks Eric)
> v3: Fix the locking too (Eric Dumazet)
> v4: Use a mutex. Get rid of retry loop.
> idr_pre_get() is still there to avoid major surgery in lib/idr.c.
> Random gleixnerfication
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---

> +	mutex_lock(&sig->idr_lock);
> +	error = -EAGAIN;
> +	if (idr_pre_get(&sig->posix_timers_id, GFP_KERNEL))
> +		error = idr_get_new(&sig->posix_timers_id, new_timer, 
> +				    &new_timer_id);
> +	mutex_unlock(&sig->idr_lock);
>  	if (error) {

On 64bit arches :

IDR_FREE_MAX=12 && sizeof(struct idr_layer)=544

This means idr_pre_get() consumes 6528 bytes of spare space per process,
even if only one posix timer is used.

I guess we should allow idr to call slab directly in this case (only a
mutex held, it's safe to sleep in slab), and not have this per idr
reserve/free_list : This was done because of some idr callers using a
spinlock.

I dont believe its major idr surgery :)



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