From: Jiayuan Chen In paddr mode with large nr_regions (20k+), damon_get_folio() dominates kdamond CPU time. perf annotate shows ~98% of its samples on a single load: reading page->compound_head for page_folio(). DAMON samples a random PFN per region, so the access pattern scatters across the vmemmap with no locality that hardware prefetchers can exploit; every compound_head load misses to DRAM (~250 cycles). Issue a software prefetch for the next region's struct page one iteration before it is needed. In __damon_pa_prepare_access_check(), the next region's sampling_addr is picked eagerly (one iteration ahead of where its mkold will run) so the prefetched line is usable, and the first region of each epoch is handled with a one-off branch since it has no predecessor to have pre-picked its addr. damon_pa_check_accesses() just prefetches based on sampling_addr already populated by the preceding prepare pass. Two details worth calling out: - prefetchw() rather than prefetch(): compound_head (+0x8) and _refcount (+0x34) share a 64B cacheline. The subsequent folio_try_get() / folio_put() atomics write _refcount, so bringing the line in exclusive state avoids a later S->M coherence upgrade. - pfn_to_page() rather than pfn_to_online_page(): on CONFIG_SPARSEMEM_VMEMMAP the former is pure arithmetic (vmemmap + pfn), while the latter walks the mem_section table. The mem_section lookup itself incurs a DRAM miss for random PFNs, which would serialize what is supposed to be a non-blocking hint - an earlier attempt that used pfn_to_online_page() saw the prefetch path's internal stall dominate perf (~91% skid on the converge point after the call). Prefetching an unmapped vmemmap entry is safe: the hint is dropped without faulting. Concurrency: list_next_entry() and &list == &head comparisons are safe without locking because kdamond is the sole mutator of the region list of its ctx; external threads must go through damon_call() which defers execution to the same kdamond thread between sampling iterations (see documentation on struct damon_ctx and damon_call()). Tested with paddr monitoring, max_nr_regions=20000, and stress-ng-vm consuming ~90% of memory across 8 workers: kdamond CPU further reduced from ~50% to ~40% of one core on top of the earlier damon_rand_fast() change. Cc: Jiayuan Chen Signed-off-by: Jiayuan Chen --- mm/damon/paddr.c | 31 ++++++++++++++++++++++++++++--- 1 file changed, 28 insertions(+), 3 deletions(-) diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c index b5e1197f2ba2..99bdf2b88cf1 100644 --- a/mm/damon/paddr.c +++ b/mm/damon/paddr.c @@ -10,6 +10,7 @@ #include #include #include +#include #include #include #include @@ -48,10 +49,29 @@ static void damon_pa_mkold(phys_addr_t paddr) folio_put(folio); } +static void damon_pa_prefetch_page(unsigned long sampling_addr, + unsigned long addr_unit) +{ + phys_addr_t paddr = damon_pa_phys_addr(sampling_addr, addr_unit); + + prefetchw(pfn_to_page(PHYS_PFN(paddr))); +} + static void __damon_pa_prepare_access_check(struct damon_ctx *ctx, + struct damon_target *t, struct damon_region *r) { - r->sampling_addr = damon_rand_fast(ctx, r->ar.start, r->ar.end); + struct damon_region *next = list_next_entry(r, list); + + /* First region has no predecessor to have pre-picked its addr. */ + if (r->list.prev == &t->regions_list) + r->sampling_addr = damon_rand_fast(ctx, r->ar.start, r->ar.end); + + if (&next->list != &t->regions_list) { + next->sampling_addr = damon_rand_fast(ctx, next->ar.start, + next->ar.end); + damon_pa_prefetch_page(next->sampling_addr, ctx->addr_unit); + } damon_pa_mkold(damon_pa_phys_addr(r->sampling_addr, ctx->addr_unit)); } @@ -63,7 +83,7 @@ static void damon_pa_prepare_access_checks(struct damon_ctx *ctx) damon_for_each_target(t, ctx) { damon_for_each_region(r, t) - __damon_pa_prepare_access_check(ctx, r); + __damon_pa_prepare_access_check(ctx, t, r); } } @@ -106,11 +126,16 @@ static void __damon_pa_check_access(struct damon_region *r, static unsigned int damon_pa_check_accesses(struct damon_ctx *ctx) { struct damon_target *t; - struct damon_region *r; + struct damon_region *r, *next; unsigned int max_nr_accesses = 0; damon_for_each_target(t, ctx) { damon_for_each_region(r, t) { + next = list_next_entry(r, list); + if (&next->list != &t->regions_list) + damon_pa_prefetch_page(next->sampling_addr, + ctx->addr_unit); + __damon_pa_check_access( r, &ctx->attrs, ctx->addr_unit); max_nr_accesses = max(r->nr_accesses, max_nr_accesses); -- 2.43.0