On Tue, May 25, 2021 at 01:39:29AM +0200, Vlastimil Babka wrote: > Currently __slab_alloc() disables irqs around the whole ___slab_alloc(). This > includes cases where this is not needed, such as when the allocation ends up in > the page allocator and has to awkwardly enable irqs back based on gfp flags. > Also the whole kmem_cache_alloc_bulk() is executed with irqs disabled even when > it hits the __slab_alloc() slow path, and long periods with disabled interrupts > are undesirable. > > As a first step towards reducing irq disabled periods, move irq handling into > ___slab_alloc(). Callers will instead prevent the s->cpu_slab percpu pointer > from becoming invalid via migrate_disable(). This does not protect against > access preemption, which is still done by disabled irq for most of > ___slab_alloc(). As the small immediate benefit, slab_out_of_memory() call from > ___slab_alloc() is now done with irqs enabled. > > kmem_cache_alloc_bulk() disables irqs for its fastpath and then re-enables them > before calling ___slab_alloc(), which then disables them at its discretion. The > whole kmem_cache_alloc_bulk() operation also disables cpu migration. > > When ___slab_alloc() calls new_slab() to allocate a new page, re-enable > preemption, because new_slab() will re-enable interrupts in contexts that allow > blocking. > > The patch itself will thus increase overhead a bit due to disabled migration > and increased disabling/enabling irqs in kmem_cache_alloc_bulk(), but that will > be gradually improved in the following patches. > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> Why did you use migrate_disable instead of preempt_disable? There is a fairly large comment in include/linux/preempt.h on why migrate_disable is undesirable so new users are likely to be put under the microscope once Thomas or Peter notice it. I think you are using it so that an allocation request can be preempted by a higher priority task but given that the code was disabling interrupts, there was already some preemption latency. However, migrate_disable is more expensive than preempt_disable (function call versus a simple increment). On that basis, I'd recommend starting with preempt_disable and only using migrate_disable if necessary. Bonus points for adding a comment where ___slab_alloc disables IRQs to clarify what is protected -- I assume it's protecting kmem_cache_cpu from being modified from interrupt context. If so, it's potentially a local_lock candidate. -- Mel Gorman SUSE Labs
