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The current implementation of swap cgroup tracking is a bit complex and fragile: On charging path, swap_cgroup_record always records an actual memcg id, and it depends on the caller to make sure all entries passed in must belong to one single folio. As folios are always charged or uncharged as a whole, and always charged and uncharged in order, swap_cgroup doesn't need an extra lock. On uncharging path, swap_cgroup_record always sets the record to zero. These entries won't be charged again until uncharging is done. So there is no extra lock needed either. Worth noting that swap cgroup clearing may happen without folio involved, eg. exiting processes will zap its page table without swapin. The xchg/cmpxchg provides atomic operations and barriers to ensure no tearing or synchronization issue of these swap cgroup records. It works but quite error-prone. Things can be much clear and robust by decoupling recording and clearing into two helpers. Recording takes the actual folio being charged as argument, and clearing always set the record to zero, and refine the debug sanity checks to better reflect their usage Benchmark even showed a very slight improvement as it saved some extra arguments and lookups: make -j96 with defconfig on tmpfs in 1.5G memory cgroup using 4k folios: Before: sys 9617.23 (stdev 37.764062) After : sys 9541.54 (stdev 42.973976) make -j96 with defconfig on tmpfs in 2G memory cgroup using 64k folios: Before: sys 7358.98 (stdev 54.927593) After : sys 7337.82 (stdev 39.398956) Link: https://lkml.kernel.org/r/20241218114633.85196-5-ryncsn@gmail.com Signed-off-by: Kairui Song <kasong@tencent.com> Suggested-by: Chris Li <chrisl@kernel.org> Cc: Barry Song <baohua@kernel.org> Cc: Hugh Dickins <hughd@google.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Roman Gushchin <roman.gushchin@linux.dev> Cc: Shakeel Butt <shakeel.butt@linux.dev> Cc: Yosry Ahmed <yosryahmed@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
48 lines
966 B
C
48 lines
966 B
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __LINUX_SWAP_CGROUP_H
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#define __LINUX_SWAP_CGROUP_H
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#include <linux/swap.h>
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#if defined(CONFIG_MEMCG) && defined(CONFIG_SWAP)
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extern void swap_cgroup_record(struct folio *folio, swp_entry_t ent);
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extern unsigned short swap_cgroup_clear(swp_entry_t ent, unsigned int nr_ents);
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extern unsigned short lookup_swap_cgroup_id(swp_entry_t ent);
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extern int swap_cgroup_swapon(int type, unsigned long max_pages);
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extern void swap_cgroup_swapoff(int type);
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#else
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static inline
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void swap_cgroup_record(struct folio *folio, swp_entry_t ent)
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{
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}
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static inline
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unsigned short swap_cgroup_clear(swp_entry_t ent, unsigned int nr_ents)
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{
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return 0;
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}
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static inline
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unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
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{
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return 0;
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}
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static inline int
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swap_cgroup_swapon(int type, unsigned long max_pages)
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{
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return 0;
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}
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static inline void swap_cgroup_swapoff(int type)
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{
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return;
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}
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#endif
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#endif /* __LINUX_SWAP_CGROUP_H */
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