Re: [PATCH v4 3/9] slab: sheaf prefilling for guaranteed allocations

[Vlastimil Babka <vbabka@suse.cz>]

On 5/7/25 11:15, Harry Yoo wrote:
> On Fri, Apr 25, 2025 at 10:27:23AM +0200, Vlastimil Babka wrote:
>> Add functions for efficient guaranteed allocations e.g. in a critical
>> section that cannot sleep, when the exact number of allocations is not
>> known beforehand, but an upper limit can be calculated.
>> 
>> kmem_cache_prefill_sheaf() returns a sheaf containing at least given
>> number of objects.
>> 
>> kmem_cache_alloc_from_sheaf() will allocate an object from the sheaf
>> and is guaranteed not to fail until depleted.
>> 
>> kmem_cache_return_sheaf() is for giving the sheaf back to the slab
>> allocator after the critical section. This will also attempt to refill
>> it to cache's sheaf capacity for better efficiency of sheaves handling,
>> but it's not stricly necessary to succeed.
>> 
>> kmem_cache_refill_sheaf() can be used to refill a previously obtained
>> sheaf to requested size. If the current size is sufficient, it does
>> nothing. If the requested size exceeds cache's sheaf_capacity and the
>> sheaf's current capacity, the sheaf will be replaced with a new one,
>> hence the indirect pointer parameter.
>> 
>> kmem_cache_sheaf_size() can be used to query the current size.
>> 
>> The implementation supports requesting sizes that exceed cache's
>> sheaf_capacity, but it is not efficient - such "oversize" sheaves are
>> allocated fresh in kmem_cache_prefill_sheaf() and flushed and freed
>> immediately by kmem_cache_return_sheaf(). kmem_cache_refill_sheaf()
>> might be especially ineffective when replacing a sheaf with a new one of
>> a larger capacity. It is therefore better to size cache's
>> sheaf_capacity accordingly to make oversize sheaves exceptional.
>> 
>> CONFIG_SLUB_STATS counters are added for sheaf prefill and return
>> operations. A prefill or return is considered _fast when it is able to
>> grab or return a percpu spare sheaf (even if the sheaf needs a refill to
>> satisfy the request, as those should amortize over time), and _slow
>> otherwise (when the barn or even sheaf allocation/freeing has to be
>> involved). sheaf_prefill_oversize is provided to determine how many
>> prefills were oversize (counter for oversize returns is not necessary as
>> all oversize refills result in oversize returns).
>> 
>> When slub_debug is enabled for a cache with sheaves, no percpu sheaves
>> exist for it, but the prefill functionality is still provided simply by
>> all prefilled sheaves becoming oversize. If percpu sheaves are not
>> created for a cache due to not passing the sheaf_capacity argument on
>> cache creation, the prefills also work through oversize sheaves, but
>> there's a WARN_ON_ONCE() to indicate the omission.
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>> ---
> 
> Looks good to me,
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> 
> with a nit below.

Thanks, incorporated the suggestion!