From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6CEF81FBC84 for ; Tue, 11 Feb 2025 11:38:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739273912; cv=none; b=e9fE1KZFnajjNIqh3ruvAXqBJ61lMBAmNGW4Ov1jj7CAcmfqEV8Ka86VmQSMV/Tealh+apSskM62zRsHT8McG9hc3tPbpib3ax1mhzLEMI95Og/AjITY6PzeYCg0Cxk7QU39ahXsnonRs3Z1gvOg7EqKRRMC73f9ul9Ovi7naEE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739273912; c=relaxed/simple; bh=0R0mpq1SdIPVOCvZFYT3a1Gkl1tNQhFBY5fIA1aH4lk=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=TKDS9U5LZij92fTgBsBVEQ0JaSpZIW6D8cyCy7QprZUpvCZCdEi3+YXvHI7hThR9ep/DEsLmPeJd+HaEcbVeKCFfCwfd9d/cfILR8vj6ijS+lXEcV0UlWezPgY7eC8Pg/Y8IAWlOACBomAf7UCu/yeIE1wJA3Mk7hpFmuXDKBl8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=aPdr2qxu; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="aPdr2qxu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 41D29C4CEE2; Tue, 11 Feb 2025 11:38:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1739273911; bh=0R0mpq1SdIPVOCvZFYT3a1Gkl1tNQhFBY5fIA1aH4lk=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=aPdr2qxuwr67HwIcwT/lC6CXRbksK3VCKz1Q7npj50B/TlTRj+CowvrDZiXu0psUF En+KRKkoNKzHMzdZ3Ho0lnhWiCqUVWz+pvRJ3Otcli4eq6joVFCMEeCfLeIubrpEG5 Y2Wm3awA0t5usrvhS1nzHneYxcKaZh0RkL/OEpmya7tnN4v/F6l4RG5QsqXRRRLZBu W5NSJLRNvW+Q7XAtfxFV5LhMdyb4c2e0RO/eqpGfIV17A2vu9S0XLqeCqccZOZ8MMb 3pTV/K/S280s9v9vMgwNsbo4BeZCuhVWBeFoKk6gT9jnA8nCmzmmkW+Zar6WqQ3Kyz UgXmUF89EbJ8Q== Message-ID: <2bda0fa87eb266a93f81ea3116fafb930ed155af.camel@kernel.org> Subject: Re: [PATCH 4/6] nfsd: filecache: introduce NFSD_FILE_RECENT From: Jeff Layton To: Dave Chinner Cc: NeilBrown , Chuck Lever , linux-nfs@vger.kernel.org, Olga Kornievskaia , Dai Ngo , Tom Talpey Date: Tue, 11 Feb 2025 06:38:30 -0500 In-Reply-To: References: <20250207051701.3467505-1-neilb@suse.de> <20250207051701.3467505-5-neilb@suse.de> <5e6060d79e247a7e97443f200399061da8d558f9.camel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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09:01:41AM -0500, Jeff Layton wrote: > > On Fri, 2025-02-07 at 16:15 +1100, NeilBrown wrote: > > > The filecache lru is walked in 2 circumstances for 2 different reason= s. > > >=20 > > > 1/ When called from the shrinker we want to discard the first few > > > entries on the list, ignoring any with NFSD_FILE_REFERENCED set > > > because they should really be at the end of the LRU as they have b= een > > > referenced recently. So those ones are ROTATED. > > >=20 > > > 2/ When called from the nfsd_file_gc() timer function we want to disc= ard > > > anything that hasn't been used since before the previous call, and > > > mark everything else as unused at this point in time. > > >=20 > > > Using the same flag for both of these can result in some unexpected > > > outcomes. If the shrinker callback clears NFSD_FILE_REFERENCED then = the > > > nfsd_file_gc() will think the file hasn't been used in a while, while > > > really it has. > > >=20 > > > I think it is easier to reason about the behaviour if we instead have > > > two flags. > > >=20 > > > NFSD_FILE_REFERENCED means "this should be at the end of the LRU, pl= ease > > > put it there when convenient" > > > NFSD_FILE_RECENT means "this has been used recently - since the last > > > run of nfsd_file_gc() > > >=20 > > > When either caller finds an NFSD_FILE_REFERENCED entry, that entry > > > should be moved to the end of the LRU and the flag cleared. This can > > > safely happen at any time. The actual order on the lru might not be > > > strictly least-recently-used, but that is normal for linux lrus. > > >=20 > > > The shrinker callback can ignore the "recent" flag. If it ends up > > > freeing something that is "recent" that simply means that memory > > > pressure is sufficient to limit the acceptable cache age to less than > > > the nfsd_file_gc frequency. > > >=20 > > > The gc caller should primarily focus on NFSD_FILE_RECENT. It should > > > free everything that doesn't have this flag set, and should clear the > > > flag on everything else. When it clears the flag it is convenient to > > > clear the "REFERENCED" flag and move to the end of the LRU too. > > >=20 > > > With this, calls from the shrinker do not prematurely age files. It > > > will focus only on freeing those that are least recently used. > > >=20 > > > Signed-off-by: NeilBrown > > > --- > > > fs/nfsd/filecache.c | 21 +++++++++++++++++++-- > > > fs/nfsd/filecache.h | 1 + > > > fs/nfsd/trace.h | 3 +++ > > > 3 files changed, 23 insertions(+), 2 deletions(-) > > >=20 > > > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c > > > index 04588c03bdfe..9faf469354a5 100644 > > > --- a/fs/nfsd/filecache.c > > > +++ b/fs/nfsd/filecache.c > > > @@ -318,10 +318,10 @@ nfsd_file_check_writeback(struct nfsd_file *nf) > > > mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK); > > > } > > > =20 > > > - > > > static bool nfsd_file_lru_add(struct nfsd_file *nf) > > > { > > > set_bit(NFSD_FILE_REFERENCED, &nf->nf_flags); > > > + set_bit(NFSD_FILE_RECENT, &nf->nf_flags); > >=20 > > Technically, I don't think you need the REFERENCED bit at all. This is > > the only place it's set, and below this is calling list_lru_add_obj(). > > That returns false if the object was already on a per-node LRU. > >=20 > > Instead of that, you could add a list_lru helper that will rotate the > > object to the end of its nodelist if it's already on one. OTOH, that > > might mean more cross NUMA-node accesses to the spinlocks than we get > > by using a flag and doing this at GC time. >=20 > No, please don't. >=20 > Per-object reference bits are required to enable lazy LRU rotation. > The LRU lists are -hot- objects; touching them every time we touch > an object on the LRU is prohibitively expensive because of exclusive > lock/cacheline contention. Hence we defer operations like rotation > to a context where we already have the list locked and cached > exclusively for some other reason (i.e. memory reclaim). >=20 > This is the same reason we use lazy removal from LRUs - it avoids > LRU list manipulations every time a hot cached object is accessed > and/or dropped. >=20 > IOWs, removing the per-object NFSD_FILE_REFERENCED bit will undo one > of the necessary the optimisations that allow hot caches LRU > management to work efficiently with minimal overhead. >=20 Yep, that was the point of my "OTOH" comment. Keeping the REFERENCED flag is better from a "let's minimize cacheline invalidations" standpoint. --=20 Jeff Layton