RE: [RFC PATCH v1 09/13] mm: zswap: Config variable to enable compress batching in zswap_store().

["Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com>]

> -----Original Message-----
> From: Yosry Ahmed <yosryahmed@google.com>
> Sent: Wednesday, October 23, 2024 11:12 AM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> hannes@cmpxchg.org; nphamcs@gmail.com; chengming.zhou@linux.dev;
> usamaarif642@gmail.com; ryan.roberts@arm.com; Huang, Ying
> <ying.huang@intel.com>; 21cnbao@gmail.com; akpm@linux-foundation.org;
> linux-crypto@vger.kernel.org; herbert@gondor.apana.org.au;
> davem@davemloft.net; clabbe@baylibre.com; ardb@kernel.org;
> ebiggers@google.com; surenb@google.com; Accardi, Kristen C
> <kristen.c.accardi@intel.com>; zanussi@kernel.org; viro@zeniv.linux.org.uk;
> brauner@kernel.org; jack@suse.cz; mcgrof@kernel.org; kees@kernel.org;
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> fsdevel@vger.kernel.org; Feghali, Wajdi K <wajdi.k.feghali@intel.com>; Gopal,
> Vinodh <vinodh.gopal@intel.com>
> Subject: Re: [RFC PATCH v1 09/13] mm: zswap: Config variable to enable
> compress batching in zswap_store().
> 
> On Tue, Oct 22, 2024 at 7:17 PM Sridhar, Kanchana P
> <kanchana.p.sridhar@intel.com> wrote:
> >
> >
> > > -----Original Message-----
> > > From: Yosry Ahmed <yosryahmed@google.com>
> > > Sent: Tuesday, October 22, 2024 5:50 PM
> > > To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> > > Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > > hannes@cmpxchg.org; nphamcs@gmail.com;
> chengming.zhou@linux.dev;
> > > usamaarif642@gmail.com; ryan.roberts@arm.com; Huang, Ying
> > > <ying.huang@intel.com>; 21cnbao@gmail.com; akpm@linux-
> foundation.org;
> > > linux-crypto@vger.kernel.org; herbert@gondor.apana.org.au;
> > > davem@davemloft.net; clabbe@baylibre.com; ardb@kernel.org;
> > > ebiggers@google.com; surenb@google.com; Accardi, Kristen C
> > > <kristen.c.accardi@intel.com>; zanussi@kernel.org;
> viro@zeniv.linux.org.uk;
> > > brauner@kernel.org; jack@suse.cz; mcgrof@kernel.org;
> kees@kernel.org;
> > > joel.granados@kernel.org; bfoster@redhat.com; willy@infradead.org;
> linux-
> > > fsdevel@vger.kernel.org; Feghali, Wajdi K <wajdi.k.feghali@intel.com>;
> Gopal,
> > > Vinodh <vinodh.gopal@intel.com>
> > > Subject: Re: [RFC PATCH v1 09/13] mm: zswap: Config variable to enable
> > > compress batching in zswap_store().
> > >
> > > On Thu, Oct 17, 2024 at 11:41 PM Kanchana P Sridhar
> > > <kanchana.p.sridhar@intel.com> wrote:
> > > >
> > > > Add a new zswap config variable that controls whether zswap_store()
> will
> > > > compress a batch of pages, for instance, the pages in a large folio:
> > > >
> > > >   CONFIG_ZSWAP_STORE_BATCHING_ENABLED
> > > >
> > > > The existing CONFIG_CRYPTO_DEV_IAA_CRYPTO variable added in
> commit
> > > > ea7a5cbb4369 ("crypto: iaa - Add Intel IAA Compression Accelerator
> crypto
> > > > driver core") is used to detect if the system has the Intel Analytics
> > > > Accelerator (IAA), and the iaa_crypto module is available. If so, the
> > > > kernel build will prompt for
> CONFIG_ZSWAP_STORE_BATCHING_ENABLED.
> > > Hence,
> > > > users have the ability to set
> > > CONFIG_ZSWAP_STORE_BATCHING_ENABLED="y" only
> > > > on systems that have Intel IAA.
> > > >
> > > > If CONFIG_ZSWAP_STORE_BATCHING_ENABLED is enabled, and IAA is
> > > configured
> > > > as the zswap compressor, zswap_store() will process the pages in a large
> > > > folio in batches, i.e., multiple pages at a time. Pages in a batch will be
> > > > compressed in parallel in hardware, then stored. On systems without
> Intel
> > > > IAA and/or if zswap uses software compressors, pages in the batch will
> be
> > > > compressed sequentially and stored.
> > > >
> > > > The patch also implements a zswap API that returns the status of this
> > > > config variable.
> > >
> > > If we are compressing a large folio and batching is an option, is not
> > > batching ever the correct thing to do? Why is the config option
> > > needed?
> >
> > Thanks Yosry, for the code review comments! This is a good point. The main
> > consideration here was not to impact software compressors run on non-
> Intel
> > platforms, and only incur the memory footprint cost of multiple
> > acomp_req/buffers in "struct crypto_acomp_ctx" if there is IAA to reduce
> > latency with parallel compressions.
> >
> > If the memory footprint cost if acceptable, there is no reason not to do
> > batching, even if compressions are sequential. We could amortize cost
> > of the cgroup charging/objcg/stats updates.
> 
> Hmm yeah based on the next patch it seems like we allocate 7 extra
> buffers, each sized 2 * PAGE_SIZE, percpu. That's 56KB percpu (with 4K
> page size), which is non-trivial.
> 
> Making it a config option seems to be inconvenient though. Users have
> to sign up for the memory overhead if some of them won't use IAA
> batching, or disable batching all together. I would assume this would
> be especially annoying for distros, but also for anyone who wants to
> experiment with IAA batching.
> 
> The first thing that comes to mind is making this a boot option. But I
> think we can make it even more convenient and support enabling it at
> runtime. We just need to allocate the additional buffers the first
> time batching is enabled. This shouldn't be too complicated, we have
> an array of buffers on each CPU but we only allocate the first one
> initially (unless batching is enabled at boot). When batching is
> enabled, we can allocate the remaining buffers.
> 
> The only shortcoming of this approach is that if we enable batching
> then disable it, we can't free the buffers without significant
> complexity, but I think that should be fine. I don't see this being a
> common pattern.
> 
> WDYT?

Thanks for these suggestions, Yosry. Sure, let me give this a try, and share
updates.

Thanks,
Kanchana

> 
> 
> 
> >
> > Thanks,
> > Kanchana
> >
> > >
> > > >
> > > > Suggested-by: Ying Huang <ying.huang@intel.com>
> > > > Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@intel.com>
> > > > ---
> > > >  include/linux/zswap.h |  6 ++++++
> > > >  mm/Kconfig            | 12 ++++++++++++
> > > >  mm/zswap.c            | 14 ++++++++++++++
> > > >  3 files changed, 32 insertions(+)
> > > >
> > > > diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> > > > index d961ead91bf1..74ad2a24b309 100644
> > > > --- a/include/linux/zswap.h
> > > > +++ b/include/linux/zswap.h
> > > > @@ -24,6 +24,7 @@ struct zswap_lruvec_state {
> > > >         atomic_long_t nr_disk_swapins;
> > > >  };
> > > >
> > > > +bool zswap_store_batching_enabled(void);
> > > >  unsigned long zswap_total_pages(void);
> > > >  bool zswap_store(struct folio *folio);
> > > >  bool zswap_load(struct folio *folio);
> > > > @@ -39,6 +40,11 @@ bool zswap_never_enabled(void);
> > > >
> > > >  struct zswap_lruvec_state {};
> > > >
> > > > +static inline bool zswap_store_batching_enabled(void)
> > > > +{
> > > > +       return false;
> > > > +}
> > > > +
> > > >  static inline bool zswap_store(struct folio *folio)
> > > >  {
> > > >         return false;
> > > > diff --git a/mm/Kconfig b/mm/Kconfig
> > > > index 33fa51d608dc..26d1a5cee471 100644
> > > > --- a/mm/Kconfig
> > > > +++ b/mm/Kconfig
> > > > @@ -125,6 +125,18 @@ config ZSWAP_COMPRESSOR_DEFAULT
> > > >         default "zstd" if ZSWAP_COMPRESSOR_DEFAULT_ZSTD
> > > >         default ""
> > > >
> > > > +config ZSWAP_STORE_BATCHING_ENABLED
> > > > +       bool "Batching of zswap stores with Intel IAA"
> > > > +       depends on ZSWAP && CRYPTO_DEV_IAA_CRYPTO
> > > > +       default n
> > > > +       help
> > > > +       Enables zswap_store to swapout large folios in batches of 8 pages,
> > > > +       rather than a page at a time, if the system has Intel IAA for
> hardware
> > > > +       acceleration of compressions. If IAA is configured as the zswap
> > > > +       compressor, this will parallelize batch compression of upto 8 pages
> > > > +       in the folio in hardware, thereby improving large folio compression
> > > > +       throughput and reducing swapout latency.
> > > > +
> > > >  choice
> > > >         prompt "Default allocator"
> > > >         depends on ZSWAP
> > > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > > index 948c9745ee57..4893302d8c34 100644
> > > > --- a/mm/zswap.c
> > > > +++ b/mm/zswap.c
> > > > @@ -127,6 +127,15 @@ static bool zswap_shrinker_enabled =
> > > IS_ENABLED(
> > > >                 CONFIG_ZSWAP_SHRINKER_DEFAULT_ON);
> > > >  module_param_named(shrinker_enabled, zswap_shrinker_enabled,
> bool,
> > > 0644);
> > > >
> > > > +/*
> > > > + * Enable/disable batching of compressions if zswap_store is called
> with a
> > > > + * large folio. If enabled, and if IAA is the zswap compressor, pages are
> > > > + * compressed in parallel in batches of say, 8 pages.
> > > > + * If not, every page is compressed sequentially.
> > > > + */
> > > > +static bool __zswap_store_batching_enabled = IS_ENABLED(
> > > > +       CONFIG_ZSWAP_STORE_BATCHING_ENABLED);
> > > > +
> > > >  bool zswap_is_enabled(void)
> > > >  {
> > > >         return zswap_enabled;
> > > > @@ -241,6 +250,11 @@ static inline struct xarray
> > > *swap_zswap_tree(swp_entry_t swp)
> > > >         pr_debug("%s pool %s/%s\n", msg, (p)->tfm_name,         \
> > > >                  zpool_get_type((p)->zpool))
> > > >
> > > > +__always_inline bool zswap_store_batching_enabled(void)
> > > > +{
> > > > +       return __zswap_store_batching_enabled;
> > > > +}
> > > > +
> > > >  /*********************************
> > > >  * pool functions
> > > >  **********************************/
> > > > --
> > > > 2.27.0
> > > >