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Goncalves" , Andrew Morton , Mel Gorman , Michal Hocko , Thomas Gleixner Subject: Re: [PATCH] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save(). Message-ID: <20230623101111.7tuAg5p5@linutronix.de> References: <20230621143421.BgHjJklo@linutronix.de> <01031ffe-c81f-9cec-76fb-e70d548429cf@I-love.SAKURA.ne.jp> <8b6d3f39-c573-ca2b-957b-8c48c2fa68ad@I-love.SAKURA.ne.jp> <20230623094544.RcLNdg6H@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: X-Stat-Signature: u3q3jwhu3noe1uimbmqbn9ab487jiwt6 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 8733580012 X-Rspam-User: X-HE-Tag: 1687515074-983357 X-HE-Meta: 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 uONEPYxg XU2pF9Htc4xTy2F80M8Mx7+YeWNXE95/IxOjHo1rmm8K0Nmu+OYKq9n8MibFzCkYE8033gNRIOVUQ6A0za0hQCdzp7+qOdWl8Wvqg00T0lSQNuiAjQHLI9RVoRM9evDJmXDQE8EByTPr9wV0= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2023-06-23 18:51:24 [+0900], Tetsuo Handa wrote: > My understanding is that zonelist_iter_begin() accesses only zonelist_update_seq.seqcount . > From where is the zonelist_update_seq.lock accessed when zonelist_iter_begin() is called? zonelist_iter_begin() -> read_seqbegin() -> read_seqcount_begin() -> raw_read_seqcount_begin() -> __read_seqcount_begin() Now. The inner part looks like while ((__seq = seqprop_sequence(s)) & 1) cpu_relax(); but with RT enabled (in-tree, not out-of-tree, in-tree) seqprop_sequence() goes from: | __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \ | { \ | unsigned seq = READ_ONCE(s->seqcount.sequence); \ | \ | if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \ | return seq; \ | \ | if (preemptible && unlikely(seq & 1)) { \ | __SEQ_LOCK(lock_acquire); \ | __SEQ_LOCK(lockbase##_unlock(s->lock)); \ | \ | /* \ | * Re-read the sequence counter since the (possibly \ | * preempted) writer made progress. \ | */ \ | seq = READ_ONCE(s->seqcount.sequence); \ | } \ | \ | return seq; \ to | unsigned __seqprop_spinlock_sequence(const seqcount_spinlock_t *s) | { | unsigned seq = READ_ONCE(s->seqcount.sequence); | | if (unlikely(seq & 1)) { | spin_lock(s->lock); | spin_unlock(s->lock); | seq = READ_ONCE(s->seqcount.sequence); | } | return seq; | } tada \o/ Sebastian