From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx> Changes in RFC v3: - Directly use __set_bit() and __clear_bit() for the slab_node_partial flag operations to avoid exporting non-atomic "workingset" interfaces. - Change get_partial() related functions to return a slab instead of returning the freelist or single object. - Don't freeze any slab under the node list_lock to further reduce list_lock holding times, as suggested by Vlastimil Babka. - Introduce freeze_slab() to do the delay freezing and return freelist. - Reorder patches. - RFC v2: https://lore.kernel.org/all/20231021144317.3400916-1-chengming.zhou@xxxxxxxxx/ Changes in RFC v2: - Reuse PG_workingset bit to keep track of whether slub is on the per-node partial list, as suggested by Matthew Wilcox. - Fix OOM problem on kernel without CONFIG_SLUB_CPU_PARTIAL, which is caused by leak of partial slabs when get_partial_node(). - Add a patch to simplify acquire_slab(). - Reorder patches a little. - RFC v1: https://lore.kernel.org/all/20231017154439.3036608-1-chengming.zhou@xxxxxxxxx/ 1. Problem ========== Now we have to freeze the slab when get from the node partial list, and unfreeze the slab when put to the node partial list. Because we need to rely on the node list_lock to synchronize the "frozen" bit changes. This implementation has some drawbacks: - Alloc path: twice cmpxchg_double. It has to get some partial slabs from node when the allocator has used up the CPU partial slabs. So it freeze the slab (one cmpxchg_double) with node list_lock held, put those frozen slabs on its CPU partial list. Later ___slab_alloc() will cmpxchg_double try-loop again if that slab is picked to use. - Alloc path: amplified contention on node list_lock. Since we have to synchronize the "frozen" bit changes under the node list_lock, the contention of slab (struct page) can be transferred to the node list_lock. On machine with many CPUs in one node, the contention of list_lock will be amplified by all CPUs' alloc path. The current code has to workaround this problem by avoiding using cmpxchg_double try-loop, which will just break and return when contention of page encountered and the first cmpxchg_double failed. But this workaround has its own problem. For more context, see 9b1ea29bc0d7 ("Revert "mm, slub: consider rest of partial list if acquire_slab() fails""). - Free path: redundant unfreeze. __slab_free() will freeze and cache some slabs on its partial list, and flush them to the node partial list when exceed, which has to unfreeze those slabs again under the node list_lock. Actually we don't need to freeze slab on CPU partial list, in which case we can save the unfreeze cmpxchg_double operations in flush path. 2. Solution =========== We solve these problems by leaving slabs unfrozen when moving out of the node partial list and on CPU partial list, so "frozen" bit is 0. These partial slabs won't be manipulate concurrently by alloc path, the only racer is free path, which may manipulate its list when !inuse. So we need to introduce another synchronization way to avoid it, we reuse PG_workingset to keep track of whether the slab is on node partial list or not, only in that case we can manipulate the slab list. The slab will be delay frozen when it's picked to actively use by the CPU, it becomes full at the same time, in which case we still need to rely on "frozen" bit to avoid manipulating its list. So the slab will be frozen only when activate use and be unfrozen only when deactivate. 3. Testing ========== We just did some simple testing on a server with 128 CPUs (2 nodes) to compare performance for now. - perf bench sched messaging -g 5 -t -l 100000 baseline RFC 7.042s 6.966s 7.022s 7.045s 7.054s 6.985s - stress-ng --rawpkt 128 --rawpkt-ops 100000000 baseline RFC 2.42s 2.15s 2.45s 2.16s 2.44s 2.17s It shows above there is about 10% improvement on stress-ng rawpkt testcase, although no much improvement on perf sched bench testcase. Thanks for any comment and code review! Chengming Zhou (7): slub: Keep track of whether slub is on the per-node partial list slub: Prepare __slab_free() for unfrozen partial slab out of node partial list slub: Reflow ___slab_alloc() slub: Change get_partial() interfaces to return slab slub: Introduce freeze_slab() slub: Delay freezing of partial slabs slub: Optimize deactivate_slab() mm/slab.h | 19 ++++ mm/slub.c | 306 +++++++++++++++++++++++------------------------------- 2 files changed, 149 insertions(+), 176 deletions(-) -- 2.40.1
