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List-Unsubscribe: On Wed, Apr 30, 2025 at 04:26:41PM +0800, Qun-Wei Lin wrote: > This patch series introduces a new mechanism called kcompressd to > improve the efficiency of memory reclaiming in the operating system. > > Problem: > In the current system, the kswapd thread is responsible for both scanning > the LRU pages and handling memory compression tasks (such as those > involving ZSWAP/ZRAM, if enabled). This combined responsibility can lead > to significant performance bottlenecks, especially under high memory > pressure. The kswapd thread becomes a single point of contention, causing > delays in memory reclaiming and overall system performance degradation. > > Solution: > Introduced kcompressd to handle asynchronous compression during memory > reclaim, improving efficiency by offloading compression tasks from > kswapd. This allows kswapd to focus on its primary task of page reclaim > without being burdened by the additional overhead of compression. > > In our handheld devices, we found that applying this mechanism under high > memory pressure scenarios can increase the rate of pgsteal_anon per second > by over 260% compared to the situation with only kswapd. Additionally, we > observed a reduction of over 50% in page allocation stall occurrences, > further demonstrating the effectiveness of kcompressd in alleviating memory > pressure and improving system responsiveness. > > Co-developed-by: Barry Song <21cnbao@gmail.com> > Signed-off-by: Barry Song <21cnbao@gmail.com> > Signed-off-by: Qun-Wei Lin > Reference: Re: [PATCH 0/2] Improve Zram by separating compression context from kswapd - Barry Song > https://lore.kernel.org/lkml/20250313093005.13998-1-21cnbao@gmail.com/ > --- +Cc Zi Yan, who might be interested in writing a framework (or improving the existing one, padata) for parallelizing jobs (e.g. migration/compression) > include/linux/mmzone.h | 6 ++++ > mm/mm_init.c | 1 + > mm/page_io.c | 71 ++++++++++++++++++++++++++++++++++++++++++ > mm/swap.h | 6 ++++ > mm/vmscan.c | 25 +++++++++++++++ > 5 files changed, 109 insertions(+) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 6ccec1bf2896..93c9195a54ae 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -23,6 +23,7 @@ > #include > #include > #include > +#include > #include > > /* Free memory management - zoned buddy allocator. */ > @@ -1398,6 +1399,11 @@ typedef struct pglist_data { > > int kswapd_failures; /* Number of 'reclaimed == 0' runs */ > > +#define KCOMPRESS_FIFO_SIZE 256 > + wait_queue_head_t kcompressd_wait; > + struct task_struct *kcompressd; > + struct kfifo kcompress_fifo; > + > #ifdef CONFIG_COMPACTION > int kcompactd_max_order; > enum zone_type kcompactd_highest_zoneidx; > diff --git a/mm/mm_init.c b/mm/mm_init.c > index 9659689b8ace..49bae1dd4584 100644 > --- a/mm/mm_init.c > +++ b/mm/mm_init.c > @@ -1410,6 +1410,7 @@ static void __meminit pgdat_init_internals(struct pglist_data *pgdat) > pgdat_init_kcompactd(pgdat); > > init_waitqueue_head(&pgdat->kswapd_wait); > + init_waitqueue_head(&pgdat->kcompressd_wait); > init_waitqueue_head(&pgdat->pfmemalloc_wait); > > for (i = 0; i < NR_VMSCAN_THROTTLE; i++) > diff --git a/mm/page_io.c b/mm/page_io.c > index 4bce19df557b..d85deb494a6a 100644 > --- a/mm/page_io.c > +++ b/mm/page_io.c > @@ -233,6 +233,38 @@ static void swap_zeromap_folio_clear(struct folio *folio) > } > } > > +static bool swap_sched_async_compress(struct folio *folio) > +{ > + struct swap_info_struct *sis = swp_swap_info(folio->swap); > + int nid = numa_node_id(); > + pg_data_t *pgdat = NODE_DATA(nid); > + > + if (unlikely(!pgdat->kcompressd)) > + return false; > + > + if (!current_is_kswapd()) > + return false; > + > + if (!folio_test_anon(folio)) > + return false; > + /* > + * This case needs to synchronously return AOP_WRITEPAGE_ACTIVATE > + */ > + if (!mem_cgroup_zswap_writeback_enabled(folio_memcg(folio))) > + return false; > + > + sis = swp_swap_info(folio->swap); > + if (zswap_is_enabled() || data_race(sis->flags & SWP_SYNCHRONOUS_IO)) { > + if (kfifo_avail(&pgdat->kcompress_fifo) >= sizeof(folio) && > + kfifo_in(&pgdat->kcompress_fifo, &folio, sizeof(folio))) { > + wake_up_interruptible(&pgdat->kcompressd_wait); > + return true; > + } > + } > + > + return false; > +} > + > /* > * We may have stale swap cache pages in memory: notice > * them here and get rid of the unnecessary final write. > @@ -275,6 +307,15 @@ int swap_writepage(struct page *page, struct writeback_control *wbc) > */ > swap_zeromap_folio_clear(folio); > } > + > + /* > + * Compression within zswap and zram might block rmap, unmap > + * of both file and anon pages, try to do compression async > + * if possible > + */ > + if (swap_sched_async_compress(folio)) > + return 0; > + > if (zswap_store(folio)) { > count_mthp_stat(folio_order(folio), MTHP_STAT_ZSWPOUT); > folio_unlock(folio); > @@ -289,6 +330,36 @@ int swap_writepage(struct page *page, struct writeback_control *wbc) > return 0; > } > > +int kcompressd(void *p) > +{ > + pg_data_t *pgdat = (pg_data_t *)p; > + struct folio *folio; > + struct writeback_control wbc = { > + .sync_mode = WB_SYNC_NONE, > + .nr_to_write = SWAP_CLUSTER_MAX, > + .range_start = 0, > + .range_end = LLONG_MAX, > + .for_reclaim = 1, > + }; > + > + while (!kthread_should_stop()) { > + wait_event_interruptible(pgdat->kcompressd_wait, > + !kfifo_is_empty(&pgdat->kcompress_fifo)); > + > + while (!kfifo_is_empty(&pgdat->kcompress_fifo)) { > + if (kfifo_out(&pgdat->kcompress_fifo, &folio, sizeof(folio))) { > + if (zswap_store(folio)) { > + count_mthp_stat(folio_order(folio), MTHP_STAT_ZSWPOUT); > + folio_unlock(folio); > + continue; > + } > + __swap_writepage(folio, &wbc); > + } > + } > + } > + return 0; > +} > + > static inline void count_swpout_vm_event(struct folio *folio) > { > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > diff --git a/mm/swap.h b/mm/swap.h > index 6f4a3f927edb..3579da413dc2 100644 > --- a/mm/swap.h > +++ b/mm/swap.h > @@ -22,6 +22,7 @@ static inline void swap_read_unplug(struct swap_iocb *plug) > void swap_write_unplug(struct swap_iocb *sio); > int swap_writepage(struct page *page, struct writeback_control *wbc); > void __swap_writepage(struct folio *folio, struct writeback_control *wbc); > +int kcompressd(void *p); > > /* linux/mm/swap_state.c */ > /* One swap address space for each 64M swap space */ > @@ -199,6 +200,11 @@ static inline int swap_zeromap_batch(swp_entry_t entry, int max_nr, > return 0; > } > > +static inline int kcompressd(void *p) > +{ > + return 0; > +} > + > #endif /* CONFIG_SWAP */ > > #endif /* _MM_SWAP_H */ > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 3783e45bfc92..2d7b9167bfd6 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -7420,6 +7420,7 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim) > void __meminit kswapd_run(int nid) > { > pg_data_t *pgdat = NODE_DATA(nid); > + int ret; > > pgdat_kswapd_lock(pgdat); > if (!pgdat->kswapd) { > @@ -7433,7 +7434,26 @@ void __meminit kswapd_run(int nid) > } else { > wake_up_process(pgdat->kswapd); > } > + ret = kfifo_alloc(&pgdat->kcompress_fifo, > + KCOMPRESS_FIFO_SIZE * sizeof(struct folio *), > + GFP_KERNEL); > + if (ret) { > + pr_err("%s: fail to kfifo_alloc\n", __func__); > + goto out; > + } > + > + pgdat->kcompressd = kthread_create_on_node(kcompressd, pgdat, nid, > + "kcompressd%d", nid); > + if (IS_ERR(pgdat->kcompressd)) { > + pr_err("Failed to start kcompressd on node %d，ret=%ld\n", > + nid, PTR_ERR(pgdat->kcompressd)); > + pgdat->kcompressd = NULL; > + kfifo_free(&pgdat->kcompress_fifo); > + } else { > + wake_up_process(pgdat->kcompressd); > + } > } > +out: > pgdat_kswapd_unlock(pgdat); > } > > @@ -7452,6 +7472,11 @@ void __meminit kswapd_stop(int nid) > kthread_stop(kswapd); > pgdat->kswapd = NULL; > } > + if (pgdat->kcompressd) { > + kthread_stop(pgdat->kcompressd); > + pgdat->kcompressd = NULL; > + kfifo_free(&pgdat->kcompress_fifo); > + } > pgdat_kswapd_unlock(pgdat); > } > > -- > 2.45.2 > > -- Cheers, Harry / Hyeonggon