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SubjectRe: [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@hpl.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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