On Thu, Mar 28, 2024 at 1:12 AM Chengming Zhou <chengming.zhou@xxxxxxxxx> wrote: > > On 2024/3/26 07:50, Yosry Ahmed wrote: > > After the rbtree to xarray conversion, and dropping zswap_entry.refcount > > and zswap_entry.value, the only members of zswap_entry utilized by > > zero-filled pages are zswap_entry.length (always 0) and > > zswap_entry.objcg. Store the objcg pointer directly in the xarray as a > > tagged pointer and avoid allocating a zswap_entry completely for > > zero-filled pages. > > > > This simplifies the code as we no longer need to special case > > zero-length cases. We are also able to further separate the zero-filled > > pages handling logic and completely isolate them within store/load > > helpers. Handling tagged xarray pointers is handled in these two > > helpers, as well as the newly introduced helper for freeing tree > > elements, zswap_tree_free_element(). > > > > There is also a small performance improvement observed over 50 runs of > > kernel build test (kernbench) comparing the mean build time on a skylake > > machine when building the kernel in a cgroup v1 container with a 3G > > limit. This is on top of the improvement from dropping support for > > non-zero same-filled pages: > > > > base patched % diff > > real 69.915 69.757 -0.229% > > user 2956.147 2955.244 -0.031% > > sys 2594.718 2575.747 -0.731% > > > > This probably comes from avoiding the zswap_entry allocation and > > cleanup/freeing for zero-filled pages. Note that the percentage of > > zero-filled pages during this test was only around 1.5% on average. > > Practical workloads could have a larger proportion of such pages (e.g. > > Johannes observed around 10% [1]), so the performance improvement should > > be larger. > > > > This change also saves a small amount of memory due to less allocated > > zswap_entry's. In the kernel build test above, we save around 2M of > > slab usage when we swap out 3G to zswap. > > > > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@xxxxxxxxxxx/ > > > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > The code looks good, just one comment below. > > Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx> Thanks! > > > --- > > mm/zswap.c | 137 ++++++++++++++++++++++++++++++----------------------- > > 1 file changed, 78 insertions(+), 59 deletions(-) > > > > diff --git a/mm/zswap.c b/mm/zswap.c > > index 413d9242cf500..efc323bab2f22 100644 > > --- a/mm/zswap.c > > +++ b/mm/zswap.c > > @@ -183,12 +183,11 @@ static struct shrinker *zswap_shrinker; > > * struct zswap_entry > > * > [..] > > > > @@ -1531,26 +1552,27 @@ bool zswap_load(struct folio *folio) > > struct page *page = &folio->page; > > struct xarray *tree = swap_zswap_tree(swp); > > struct zswap_entry *entry; > > + struct obj_cgroup *objcg; > > + void *elem; > > > > VM_WARN_ON_ONCE(!folio_test_locked(folio)); > > > > - entry = xa_erase(tree, offset); > > - if (!entry) > > + elem = xa_erase(tree, offset); > > + if (!elem) > > return false; > > > > - if (entry->length) > > + if (!zswap_load_zero_filled(elem, page, &objcg)) { > > + entry = elem; > > nit: entry seems no use anymore. I left it here on purpose to avoid casting elem in the next two lines, it is just more aesthetic. > > > + objcg = entry->objcg; > > zswap_decompress(entry, page); > > - else > > - clear_highpage(page); > > + } > > > > count_vm_event(ZSWPIN); > > - if (entry->objcg) > > - count_objcg_event(entry->objcg, ZSWPIN); > > - > > - zswap_entry_free(entry); > > + if (objcg) > > + count_objcg_event(objcg, ZSWPIN); > > > > + zswap_tree_free_element(elem); > > folio_mark_dirty(folio); > > - > > return true; > > } > [..]
