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Message-ID: <PH7PR11MB812141865845E36F05DD388FC9832@PH7PR11MB8121.namprd11.prod.outlook.com>
Date: Wed, 30 Apr 2025 21:07:03 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
CC: Nhat Pham <nphamcs@...il.com>, lkp <lkp@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "hannes@...xchg.org"
	<hannes@...xchg.org>, "chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>,
	"usamaarif642@...il.com" <usamaarif642@...il.com>, "ryan.roberts@....com"
	<ryan.roberts@....com>, "21cnbao@...il.com" <21cnbao@...il.com>,
	"ying.huang@...ux.alibaba.com" <ying.huang@...ux.alibaba.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"davem@...emloft.net" <davem@...emloft.net>, "clabbe@...libre.com"
	<clabbe@...libre.com>, "ardb@...nel.org" <ardb@...nel.org>,
	"ebiggers@...gle.com" <ebiggers@...gle.com>, "surenb@...gle.com"
	<surenb@...gle.com>, "Accardi, Kristen C" <kristen.c.accardi@...el.com>,
	"llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
	"oe-kbuild-all@...ts.linux.dev" <oe-kbuild-all@...ts.linux.dev>, "Feghali,
 Wajdi K" <wajdi.k.feghali@...el.com>, "Gopal, Vinodh"
	<vinodh.gopal@...el.com>, "Sridhar, Kanchana P"
	<kanchana.p.sridhar@...el.com>
Subject: RE: [PATCH v8 14/14] mm: zswap: Compress batching with request
 chaining in zswap_store() of large folios.


> -----Original Message-----
> From: Yosry Ahmed <yosry.ahmed@...ux.dev>
> Sent: Thursday, March 6, 2025 1:21 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Cc: Nhat Pham <nphamcs@...il.com>; lkp <lkp@...el.com>; linux-
> kernel@...r.kernel.org; linux-mm@...ck.org; hannes@...xchg.org;
> chengming.zhou@...ux.dev; usamaarif642@...il.com;
> ryan.roberts@....com; 21cnbao@...il.com;
> ying.huang@...ux.alibaba.com; akpm@...ux-foundation.org; linux-
> crypto@...r.kernel.org; herbert@...dor.apana.org.au;
> davem@...emloft.net; clabbe@...libre.com; ardb@...nel.org;
> ebiggers@...gle.com; surenb@...gle.com; Accardi, Kristen C
> <kristen.c.accardi@...el.com>; llvm@...ts.linux.dev; oe-kbuild-
> all@...ts.linux.dev; Feghali, Wajdi K <wajdi.k.feghali@...el.com>; Gopal,
> Vinodh <vinodh.gopal@...el.com>
> Subject: Re: [PATCH v8 14/14] mm: zswap: Compress batching with request
> chaining in zswap_store() of large folios.
> 
> On Mon, Mar 03, 2025 at 09:34:04PM +0000, Sridhar, Kanchana P wrote:
> >
> > > -----Original Message-----
> > > From: Nhat Pham <nphamcs@...il.com>
> > > Sent: Monday, March 3, 2025 10:22 AM
> > > To: lkp <lkp@...el.com>
> > > Cc: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>; linux-
> > > kernel@...r.kernel.org; linux-mm@...ck.org; hannes@...xchg.org;
> > > yosry.ahmed@...ux.dev; chengming.zhou@...ux.dev;
> > > usamaarif642@...il.com; ryan.roberts@....com; 21cnbao@...il.com;
> > > ying.huang@...ux.alibaba.com; akpm@...ux-foundation.org; linux-
> > > crypto@...r.kernel.org; herbert@...dor.apana.org.au;
> > > davem@...emloft.net; clabbe@...libre.com; ardb@...nel.org;
> > > ebiggers@...gle.com; surenb@...gle.com; Accardi, Kristen C
> > > <kristen.c.accardi@...el.com>; llvm@...ts.linux.dev; oe-kbuild-
> > > all@...ts.linux.dev; Feghali, Wajdi K <wajdi.k.feghali@...el.com>; Gopal,
> > > Vinodh <vinodh.gopal@...el.com>
> > > Subject: Re: [PATCH v8 14/14] mm: zswap: Compress batching with
> request
> > > chaining in zswap_store() of large folios.
> > >
> > > On Mon, Mar 3, 2025 at 3:07 AM kernel test robot <lkp@...el.com>
> wrote:
> > > >
> > > > Hi Kanchana,
> > > >
> > > > kernel test robot noticed the following build errors:
> > > >
> > > > > 1166                          prefetchw(entries[j]);
> > > > --
> > >
> > > Why are we doing this anyway? Does it have a notable performance
> > > difference? At the very least, leave a comment explaining why we're
> > > prefetching this (although the build error suggests that we have to
> > > remove it anyway).
> >
> > Hi Nhat,
> >
> > Yes, it does. The use of prefetchw reduces sys time by ~1.5% because
> > it minimizes cache-miss latency by moving the zswap entry to the cache
> > before it is written to.
> >
> > This is data with kernel compilation test, v8 without prefetchw and v8 as-is:
> >
> > --------------------------------------------------------------------------------
> >  Kernel compile       v8 without               v8      v8 without              v8
> >  allmodconfig          prefetchw                        prefetchw
> >  2M folios
> >  --------------------------------------------------------------------------------
> >  zswap compressor    deflate-iaa      deflate-iaa            zstd            zstd
> >  --------------------------------------------------------------------------------
> >  real_sec                 732.89           735.63          768.53          758.21
> >  user_sec              15,708.37        15,699.84       15,702.64       15,678.73
> >  sys_sec                4,632.58         4,563.70        5,735.06        5,635.69
> >  --------------------------------------------------------------------------------
> >  Max_Res_Set_Size_KB   1,874,672        1,867,516       1,874,684
> 1,872,888
> >  --------------------------------------------------------------------------------
> >  memcg_high                    0                0               0               0
> >  memcg_swap_fail               0                0               0               0
> >  zswpout             114,742,930      112,836,725      92,904,961      89,596,085
> >  zswpin               41,184,897       39,983,793      31,018,149      29,163,932
> >  pswpout                     625            1,069             558           1,059
> >  pswpin                      599            1,056             540           1,051
> >  thp_swpout                    1                2               1               2
> >  thp_swpout_fallback      10,967           10,195           6,918           6,141
> >  pgmajfault           42,588,331       41,349,069      31,931,882      30,006,422
> >  ZSWPOUT-2048kB            7,661            8,710           6,799           7,480
> >  SWPOUT-2048kB                 1                2               1               2
> >  --------------------------------------------------------------------------------
> >
> >
> > Sure, I will add a comment, and also "#include <linux/prefetch.h>" in
> zswap.c
> > that will resolve the build error. This is similar to how these files handle
> prefetchw:
> > mm/vmscan.c, kernel/locking/qspinlock.c, include/asm-generic/xor.h, etc.
> 
> Please also explicitly mention that the prefetch and likely/unlikely
> annotations prevent regressions with software compression like zstd, and
> generally improve the performance with the batching code by ~1.5%.

Yes, I have mentioned this in the comments and commit log. So also the
mutex locking.

> 
> >
> > Thanks,
> > Kanchana
> >

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