linux-kernel - Re: [PATCH 8/18] 2.6.17.9 perfmon2 patch for review: event sets and multiplexing support

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Date:	Thu, 7 Sep 2006 23:26:28 -0700
From:	Stephane Eranian <eranian@....hp.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/18] 2.6.17.9 perfmon2 patch for review: event sets and multiplexing support

Andrew,

On Wed, Sep 06, 2006 at 07:21:35PM -0700, Andrew Morton wrote:
> On Wed, 6 Sep 2006 07:50:31 -0700
> Stephane Eranian <eranian@....hp.com> wrote:
> 
> > > > +
> > > > +	cachep = ctx->flags.mapset ? pfm_set_cachep : pfm_lg_set_cachep;
> > > > +
> > > > +	new_set = kmem_cache_alloc(cachep, SLAB_ATOMIC);
> > > 
> > > SLAB_ATOMIC is unreliable.  Is it possible to use SLAB_KERNEL here?  If
> > > coms ecallers can sleep and others cannot then passing in the gfp_flags
> > > would permit improvement here.
> > > 
> > 
> > I made some changes and now I know I execute this part of the function
> > with interrupts disabled, holding only the perfmon context lock. I assume
> > SLAB_KERNEL means, we can sleep. I think I can make this change safely.
> > 
> > 
> > > 
> > > > +		if (ctx->flags.mapset) {
> > > > +			view_size = PAGE_ALIGN(sizeof(struct pfm_set_view));
> > > > +			view      = vmalloc(view_size);
> > > 
> > > vmalloc() sleeps, so this _could_ have used SLAB_ATOMIC.
> > > 
> > 
> > I am not sure I follow you here. Are you talking about eh kmem_cache_alloc()
> > above?
> > 
> 
> My logic was as follows:
> 
> a) vmalloc() can sleep
> 
> b) Stephane at some time tested this conde with
>    CONFIG_DEBUG_SPINLOCK_SLEEP and didn't get sleep-while-atomic warnings out of
>    that vmalloc().
> 
> c) Hence this code is never called under spinlock, or with local
>    interrupts disabled.
> 
> d) Hence it is safe to convert the earlier SLAB_ATOMIC into SLAB_KERNEL.
> 
> 
> If b) is false then it's the vmalloc() call which is incorrect, not the
> SLAB_ATOMIC.

Looking at the code again, I now think that vmalloc is wrong. I have made
some changes to lift the restrictions on interrupts being masked, but I still
need to hold a spinlock. So I think, I need to replace vmalloc with kmalloc
and SLAB_ATOMIC. Furthermore, I think I need to surround this with a pair
of preempt_disable/preempt_enable (given the interrupts are unmasked).

Thanks.

-- 

-Stephane
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