From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Minchan Kim <minchan@kernel.org>,
linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>,
Mike Galbraith <umgwanakikbuti@gmail.com>,
Mike Galbraith <efault@gmx.de>
Subject: Re: [PATCH] mm/zsmalloc: Replace bit spinlock and get_cpu_var() usage.
Date: Wed, 29 Sep 2021 09:23:59 +0200 [thread overview]
Message-ID: <20210929072359.zkzg57gf362tc76m@linutronix.de> (raw)
In-Reply-To: <20210928154723.1b0577818143734653d9b129@linux-foundation.org>
On 2021-09-28 15:47:23 [-0700], Andrew Morton wrote:
> Rather nasty with all the ifdefs and two different locking approaches
> to be tested. What would be the impact of simply switching to the new
> scheme for all configs?
The current scheme uses the lower bit (OBJ_ALLOCATED_TAG) as something
special which is guaranteed to be zero due to memory alignment
requirements. The content of the memory, that long, is then used a bit
spinlock.
Moving it to spinlock_t would consume only 4 bytes of memory assuming
lockdep is off. It is then 4 bytes less than a long on 64 bits archs.
So we could do this if nobody disagrees. The spinlock_t has clearly
advantages over a bit spinlock like the "order" from the qspinlock
implementation. But then I have no idea what the contention here is.
With lockdep enabled the struct gets a little bigger which I assume was to
avoid. But then only debug builds are affected so…
> Which is identical to asking "what is the impact of switching to the new
> scheme for PREEMPT_RT"! Which is I think an important thing for the
> changelog to address?
Well, PREEMPT_RT can't work with the bit spinlock in it. That is that
part from the changelog:
| 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.
Sebastian
next prev parent reply other threads:[~2021-09-29 7:24 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
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 [this message]
2021-09-29 19:09 ` Minchan Kim
2021-09-30 6:42 ` Sebastian Andrzej Siewior
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