Re: SLAB cache reaper on isolated cpus

[Max Krasnyansky <maxk@qualcomm.com>]

Christoph Lameter wrote:
> On Tue, 20 Feb 2007, Max Krasnyansky wrote:
> 
>> Suppose I need to isolate a CPU. We already support at the scheduler and 
>> irq levels (irq affinity). But I want to go a bit further and avoid 
>> doing kernel work on isolated cpus as much as possible. For example I 
>> would not want to schedule work queues and stuff on them. Currently 
>> there are just a few users of the schedule_delayed_work_on(). cpufreq 
>> (don't care for isolation purposes), oprofile (same here) and slab. For 
>> the slab it'd be nice to run the reaper on some other CPU. But you're 
>> saying that locking depends on CPU pinning. Is there any other option 
>> besides disabling cache reap ? Is there a way for example to constraint 
>> the slabs on CPU X to not exceed N megs ?
> 
> There is no way to constrain the amount of slab work. In order to make the 
> above work we would have to disable the per cpu caches for a certain cpu. 
> Then there would be no need to run the cache reaper at all.
> 
> To some extend such functionality already exists. F.e. kmalloc_node() 
> already bypasses the per cpu caches (most of the time).  kmalloc_node will 
> have to take a spinlock on a shared cacheline on each invocation. kmalloc 
> does only touch per cpu data during regular operations. Thus kmalloc() is much 
> faster than kmalloc_node() and the cachelines for kmalloc() can be kept in 
> the per cpu cache.
> 
> If we could disable all per cpu caches for certain cpus then you could 
> make this work. All slab OS interference would be off the processor.

Hmm. That's an idea. I'll play with it later today or tomorrow.

Thanks
Max