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McKenney" To: Tejun Heo Cc: Josef Bacik , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jan Kara , Roman Gushchin , Dennis Zhou , "Matthew Wilcox (Oracle)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] writeback: report a Tasks-RCU quiescent state per cgwb drain pass Message-ID: Reply-To: paulmck@kernel.org References: <20260909-cgwb-tasks-rcu-qs-v1-1-967a7754771f@toxicpanda.com> <16d13239-8911-4597-bfd6-18aa02135137@paulmck-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 35A5F40004 X-Stat-Signature: bbsmgb11i65cmrmxp5bayiq4o4efaoab X-HE-Tag: 1788984781-579422 X-HE-Meta: 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 BWrWF0RY 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 09, 2026 at 09:13:49AM -1000, Tejun Heo wrote: > Hello, > > On Wed, Sep 09, 2026 at 12:03:20PM -0700, Paul E. McKenney wrote: > > > The patch looks fine but this overall seems fragile. cond_resched() was > > > already marking "stuff that can take too long" but we need to use > > > cond_resched_tasks_rcu_qs() if it can take *really* long. There gotta be a > > > way to make this more maintainable. If always doing tasks_rcu_qs from > > > cond_resched() is too expensive, can it be be gated behind something cheaper > > > e.g. some tick based test? > > > > This is the business end of cond_resched_tasks_rcu_qs() in preemptible > > kernels (in which cond_resched() is nothingness): > > > > # define rcu_tasks_classic_qs(t, preempt) \ > > do { \ > > if (!(preempt) && READ_ONCE((t)->rcu_tasks_holdout)) \ > > WRITE_ONCE((t)->rcu_tasks_holdout, false); \ > > } while (0) > > > > This is pretty lightweight. Adding a jiffies check would likely make > > it more expensive. > > > > Or am I missing your point? > > I found the following thread for why there is a separate variant for > cond_resched_tasks_rcu_qs(): > > https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home Well, that was nine years ago. ;-) > The rationale was that it'd make cond_resched() expensive, so I assumed it > was relatively heavy. If it already comes down to a single test, I'm not > sure having a separate interface makes a lot of sense. There isn't some > semantical difference between the two, right? Anything which takes long > enough needs to do the tasks rcu qs, and that is what we mark with > cond_resched(). And nine years later, I am still good with cond_resched() implying cond_resched_tasks_rcu_qs(). The people wanting to get rid of cond_resched() might have a different opinion, though. Thanx, Paul