linux-kernel - Re: [PATCH] zsmalloc: do not use bit_spin

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Date:   Mon, 21 Dec 2020 13:02:45 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
Cc:     Vitaly Wool <vitaly.wool@...sulko.com>,
        Minchan Kim <minchan@...nel.org>,
        Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        NitinGupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock

On Mon, Dec 21, 2020 at 12:06 PM Song Bao Hua (Barry Song)
<song.bao.hua@...ilicon.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Shakeel Butt [mailto:shakeelb@...gle.com]
> > Sent: Tuesday, December 22, 2020 8:50 AM
> > To: Vitaly Wool <vitaly.wool@...sulko.com>
> > Cc: Minchan Kim <minchan@...nel.org>; Mike Galbraith <efault@....de>; LKML
> > <linux-kernel@...r.kernel.org>; linux-mm <linux-mm@...ck.org>; Song Bao Hua
> > (Barry Song) <song.bao.hua@...ilicon.com>; Sebastian Andrzej Siewior
> > <bigeasy@...utronix.de>; NitinGupta <ngupta@...are.org>; Sergey Senozhatsky
> > <sergey.senozhatsky.work@...il.com>; Andrew Morton
> > <akpm@...ux-foundation.org>
> > Subject: Re: [PATCH] zsmalloc: do not use bit_spin_lock
> >
> > On Mon, Dec 21, 2020 at 11:20 AM Vitaly Wool <vitaly.wool@...sulko.com> wrote:
> > >
> > > On Mon, Dec 21, 2020 at 6:24 PM Minchan Kim <minchan@...nel.org> wrote:
> > > >
> > > > On Sun, Dec 20, 2020 at 02:22:28AM +0200, Vitaly Wool wrote:
> > > > > zsmalloc takes bit spinlock in its _map() callback and releases it
> > > > > only in unmap() which is unsafe and leads to zswap complaining
> > > > > about scheduling in atomic context.
> > > > >
> > > > > To fix that and to improve RT properties of zsmalloc, remove that
> > > > > bit spinlock completely and use a bit flag instead.
> > > >
> > > > I don't want to use such open code for the lock.
> > > >
> > > > I see from Mike's patch, recent zswap change introduced the lockdep
> > > > splat bug and you want to improve zsmalloc to fix the zswap bug and
> > > > introduce this patch with allowing preemption enabling.
> > >
> > > This understanding is upside down. The code in zswap you are referring
> > > to is not buggy.  You may claim that it is suboptimal but there is
> > > nothing wrong in taking a mutex.
> > >
> >
> > Is this suboptimal for all or just the hardware accelerators? Sorry, I
> > am not very familiar with the crypto API. If I select lzo or lz4 as a
> > zswap compressor will the [de]compression be async or sync?
>
> Right now, in crypto subsystem, new drivers are required to write based on
> async APIs. The old sync API can't work in new accelerator drivers as they
> are not supported at all.
>
> Old drivers are used to sync, but they've got async wrappers to support async
> APIs. Eg.
> crypto: acomp - add support for lz4 via scomp
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/crypto/lz4.c?id=8cd9330e0a615c931037d4def98b5ce0d540f08d
>
> crypto: acomp - add support for lzo via scomp
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/crypto/lzo.c?id=ac9d2c4b39e022d2c61486bfc33b730cfd02898e
>
> so they are supporting async APIs but they are still working in sync mode as
> those old drivers don't sleep.
>

Good to know that those are sync because I want them to be sync.
Please note that zswap is a cache in front of a real swap and the load
operation is latency sensitive as it comes in the page fault path and
directly impacts the applications. I doubt decompressing synchronously
a 4k page on a cpu will be costlier than asynchronously decompressing
the same page from hardware accelerators.