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Subject: Re: [thiscpuops upgrade 10/10] Lockless (and preemptless)
 fastpaths for slub
From: Peter Zijlstra <peterz@infradead.org>
To: Christoph Lameter <cl@linux.com>
Cc: akpm@linux-foundation.org, Pekka Enberg <penberg@cs.helsinki.fi>,
        Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
        Eric Dumazet <eric.dumazet@gmail.com>,
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        Tejun Heo <tj@kernel.org>
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Date: Wed, 24 Nov 2010 18:26:50 +0100
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On Wed, 2010-11-24 at 10:14 -0600, Christoph Lameter wrote:
> On Wed, 24 Nov 2010, Peter Zijlstra wrote:
> 
> > This thing still relies on disabling IRQs in the slow path, which means
> > its still going to be a lot of work to make it work on -rt.
> 
> The disabling of irqs is because slab operations are used from interrupt
> context. If we can avoid slab operations from interrupt contexts then we
> can drop the interrupt disable in the slab allocators.

That's not so much the point, there's per-cpu assumptions due to that.
Not everything is under a proper lock, see for example this bit:

        new = new_slab(s, gfpflags, node);

        if (gfpflags & __GFP_WAIT)
                local_irq_disable();

        if (new) {
                c = __this_cpu_ptr(s->cpu_slab);
                stat(s, ALLOC_SLAB);
                if (c->page)
                        flush_slab(s, c);
                slab_lock(new);
                __SetPageSlubFrozen(new);
                c->page = new;
                goto load_freelist;
        }

There we have the __this_cpu_ptr, c->page deref and flush_slab()->stat()
call all before we take a lock.