From: nsaenzju@redhat.com
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
akpm@linux-foundation.org, frederic@kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
tglx@linutronix.de, cl@linux.com, juri.lelli@redhat.com,
mingo@redhat.com, mtosatti@redhat.com, nilal@redhat.com,
mgorman@suse.de, ppandit@redhat.com, williams@redhat.com,
anna-maria@linutronix.de, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH 2/6] mm/swap: Introduce alternative per-cpu LRU cache locking
Date: Wed, 22 Sep 2021 11:50:07 +0200 [thread overview]
Message-ID: <7c8594d5943f99bc647951269606ae24dd1b37de.camel@redhat.com> (raw)
In-Reply-To: <20210922092039.2j6efnkhmfxuzjnx@linutronix.de>
On Wed, 2021-09-22 at 11:20 +0200, Sebastian Andrzej Siewior wrote:
> On 2021-09-22 10:47:07 [+0200], nsaenzju@redhat.com wrote:
> > > *why* use migrate_disable(), that's horrible!
> >
> > I was trying to be mindful of RT. They don't appreciate people taking spinlocks
> > just after having disabled preemption.
> >
> > I think getting local_lock(&locks->local) is my only option then. But it adds
> > an extra redundant spinlock in the RT+NOHZ_FULL case.
>
> spin_lock() does not disable preemption on PREEMPT_RT. You don't
> disables preemption on purpose or did I miss that?
Sorry my message wasn't clear. Adding code for context:
+ static inline void lru_cache_lock(struct lru_cache_locks *locks)
+ {
+ if (static_branch_unlikely(&remote_pcpu_cache_access)) {
+ /* Avoid migration between this_cpu_ptr() and spin_lock() */
+ migrate_disable();
IIUC PeterZ would've preferred that I disable preemption here. And what I meant
to explain is that, given the spinlock below, I choose migrate_disable() over
preempt_disable() to cater for RT.
+ spin_lock(this_cpu_ptr(&locks->spin));
+ } else {
So, to make both worlds happy, I think the only option left is using the
local_lock (at the expense of extra overhead in the RT+NOHZ_FULL case):
+ if (static_branch_unlikely(&remote_pcpu_cache_access)) {
+ /* Local lock avoids migration between this_cpu_ptr() and spin_lock() */
+ local_lock(&locks->local);
+ spin_lock(this_cpu_ptr(&locks->spin));
+ } else {
--
Nicolás Sáenz
next prev parent reply other threads:[~2021-09-22 9:50 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-21 16:13 [PATCH 0/6] mm: Remote LRU per-cpu pagevec cache/per-cpu page list drain support Nicolas Saenz Julienne
2021-09-21 16:13 ` [PATCH 1/6] mm/swap: Introduce lru_cpu_needs_drain() Nicolas Saenz Julienne
2021-09-21 16:13 ` [PATCH 2/6] mm/swap: Introduce alternative per-cpu LRU cache locking Nicolas Saenz Julienne
2021-09-21 22:03 ` Peter Zijlstra
2021-09-22 8:47 ` nsaenzju
2021-09-22 9:20 ` Sebastian Andrzej Siewior
2021-09-22 9:50 ` nsaenzju [this message]
2021-09-22 11:37 ` Peter Zijlstra
2021-09-22 11:43 ` nsaenzju
2021-09-21 16:13 ` [PATCH 3/6] mm/swap: Allow remote LRU cache draining Nicolas Saenz Julienne
2021-09-21 16:13 ` [PATCH 4/6] mm/page_alloc: Introduce alternative per-cpu list locking Nicolas Saenz Julienne
2021-09-21 16:13 ` [PATCH 5/6] mm/page_alloc: Allow remote per-cpu page list draining Nicolas Saenz Julienne
2021-09-21 16:13 ` [PATCH 6/6] sched/isolation: Enable 'remote_pcpu_cache_access' on NOHZ_FULL systems Nicolas Saenz Julienne
2021-09-21 17:51 ` [PATCH 0/6] mm: Remote LRU per-cpu pagevec cache/per-cpu page list drain support Andrew Morton
2021-09-21 17:59 ` Vlastimil Babka
2021-09-22 11:28 ` Peter Zijlstra
2021-09-22 22:09 ` Thomas Gleixner
2021-09-23 7:12 ` Vlastimil Babka
2021-09-23 10:36 ` Thomas Gleixner
2021-09-27 9:30 ` nsaenzju
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