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 b6EnGGpJ 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 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: List-Subscribe: List-Unsubscribe: Dmitry Torokhov writes: > Hi Anhur, > > On Tue, Nov 07, 2023 at 03:08:21PM -0800, Ankur Arora wrote: >> There are broadly three sets of uses of cond_resched(): >> >> 1. Calls to cond_resched() out of the goodness of our heart, >> otherwise known as avoiding lockup splats. > > ... > > What about RCU stalls? The calls to cond_resched() in evdev.c and > mousedev.c were added specifically to allow RCU to run in cases when > userspace passes a large buffer and the kernel is not fully preemptable. Hi Dmitry The short answer is that even if the kernel isn't fully preemptible, it will always have preempt-count which means that RCU will always know when a read-side critical section gets over. Long version: cond_resched_rcu() is defined as: static inline void cond_resched_rcu(void) { #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU) rcu_read_unlock(); cond_resched(); rcu_read_lock(); #endif } So the relevant case is PREEMPT_RCU=n. Now, currently PREEMPT_RCU=n, also implies PREEMPT_COUNT=n. And so the rcu_read_lock()/_unlock() reduce to a barrier. And, that's why we need the explicit cond_resched() there. The reason we can remove the cond_resched() after patch 43, and 47 is because rcu_read_lock()/_unlock() will modify the preempt count and so RCU will have visibility into when RCU read-side critical sections finish. That said, this series in this form isn't really going anywhere in the short-term so none of this is imminent. On the calls to cond_resched(), if the kernel is fully preemptible they are a NOP. And then the code would be polling in a tight loop. Would it make sense to do something like this instead? if (!cond_resched()) msleep()/usleep()/cpu_relax(); -- ankur