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From: Chengming Zhou In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: B8088C0004 X-Stat-Signature: 86sg6hhh8pyuu9sbjpqwwneageny6s8m X-HE-Tag: 1728704007-62542 X-HE-Meta: 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 4T6qAb/8 K0oEbZjJxmUwsIZ6u6+Y26BFartkSadU84s82KiD22D6af7Og8HfZibsrwO7RxEKsj4Xdq84T/9zzkYGdOmXY0hqSAi1oN1dG++23YPjtq6vksq1mwg8VOYy7Q6gebwJPSi/lRst8L10HcJNvutB1J0HJGJOTJdQAdItlEpT66jNPEutLu31rRlA7bNIyDlPDZuYtzfYuxEILIlQjzrOtx+fogqNpnNIDxoxIkOLNLt+CvQ1fpi7Ox3qYM+gDRlosPrsptfh7yXOXuTeECIFhzPRylQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2024/10/12 11:04, Kairui Song wrote: > Johannes Weiner 于 2024年10月12日周六 02:28写道： >> >> On Fri, Oct 11, 2024 at 10:53:31AM -0700, Yosry Ahmed wrote: >>> On Fri, Oct 11, 2024 at 10:20 AM Kairui Song wrote: >>>> >>>> From: Kairui Song >>>> >>>> zswap_invalidation simply calls xa_erase, which acquires the Xarray >>>> lock first, then does a look up. This has a higher overhead even if >>>> zswap is not used or the tree is empty. >>>> >>>> So instead, do a very lightweight xa_empty check first, if there is >>>> nothing to erase, don't touch the lock or the tree. >> >> Great idea! >> >>> XA_STATE(xas, ..); >>> >>> rcu_read_lock(); >>> entry = xas_load(&xas); >>> if (entry) { >>> xas_lock(&xas); >>> WARN_ON_ONCE(xas_reload(&xas) != entry); >>> xas_store(&xas, NULL); >>> xas_unlock(&xas); >>> } >>> rcu_read_unlock(): >> >> This does the optimization more reliably, and I think we should go >> with this version. > > Hi Yosry and Johannes, > > This is a good idea. But xa_empty is just much lighweighter, it's just > a inlined ( == NULL ) check, so unsurprising it has better performance > than xas_load. > > And surprisingly it's faster than zswap_never_enabled. So I think it Do you have CONFIG_ZSWAP_DEFAULT_ON enabled? In your case, CPU will go to the unlikely branch of static_key every time, which maybe the cause. > could be doable to introduce something like zswap_may_have_swpentry as > Yosry suggested. > > So how about a combined version with xas_load and xa_empty? Check > xa_empty first as a faster path, then xas_load, then xas_store. Yeah, I also think this combined version is better. Thanks. > > Here is the benchmark result (time of swapin 2G zero pages in us): > > Before: 1908944 1905870 1905322 1905627 1901667 > xa_empty: 1835343 1827367 1828402 1831841 1832719 > z.._enabled: 1838428 1831162 1838205 1837287 1840980 > xas_load: 1874606 1878971 1870182 1875852 1873403 > combined: 1845309 1832919 1831904 1836455 1842570 > > `combined` is xa_empty + xas_load.