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SubjectRe: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub
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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.




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