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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Minchan Kim <minchan@kernel.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] mm: Disable zsmalloc on PREEMPT_RT
Date: Fri, 24 Sep 2021 09:08:25 +0200	[thread overview]
Message-ID: <20210924070825.zqsi7hxgtojd67wv@linutronix.de> (raw)
In-Reply-To: <YU0IkoeWYLdll6/+@google.com>

On 2021-09-23 16:06:58 [-0700], Minchan Kim wrote:
> On Thu, Sep 23, 2021 at 07:01:21PM +0200, Sebastian Andrzej Siewior wrote:
> > For efficiency reasons, zsmalloc is using a slim `handle'. The value is
> > the address of a memory allocation of 4 or 8 bytes depending on the size
> > of the long data type. The lowest bit in that allocated memory is used
> > as a bit spin lock.
> > The usage of the bit spin lock is problematic because with the bit spin
> > lock held zsmalloc acquires a rwlock_t and spinlock_t which are both
> > sleeping locks on PREEMPT_RT and therefore must not be acquired with
> > disabled preemption.
> 
> I am not sure how long the preemption disabled section takes since it
> just disable for a page copy mostly.

after the bit-spin-lock, there are sleeping locks. These are
problematic.

> > 
> > There is a patch which extends the handle on PREEMPT_RT so that a full
> > spinlock_t fits (even with lockdep enabled) and then eliminates the bit
> > spin lock. I'm not sure how sensible zsmalloc on PREEMPT_RT is given
> > that it is used to store compressed user memory.
> 
> I don't see what's relation between PREEMPT_RT and compressed user
> memory so you can reach such conclustion. Disable zsmalloc also makes
> disable zram. I think in-compress memory swap rather than storage
> swap/block sometimes would be useful for even RT.

The only user of zsmalloc I found was zswap which compressed user pages.
Maybe I didn't collect all the dots.

> > Disable ZSMALLOC on PREEMPT_RT. If there is need for it, we can try to
> > get it to work.
> 
> Please send the patch which extends handle with spin_lock rather than
> simply disabing.

Okay. I will clean it up a little and post it then.

Sebastian


  reply	other threads:[~2021-09-24  7:08 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-23 17:01 [PATCH] mm: Disable zsmalloc on PREEMPT_RT Sebastian Andrzej Siewior
2021-09-23 23:06 ` Minchan Kim
2021-09-24  7:08   ` Sebastian Andrzej Siewior [this message]
2021-09-28  8:44   ` [PATCH] mm/zsmalloc: Replace bit spinlock and get_cpu_var() usage Sebastian Andrzej Siewior
2021-09-28 22:47     ` Andrew Morton
2021-09-29  2:11       ` Mike Galbraith
2021-09-29  7:23       ` Sebastian Andrzej Siewior
2021-09-29 19:09         ` Minchan Kim
2021-09-30  6:42           ` Sebastian Andrzej Siewior

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