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B0JpaQ0a7c/HFh0KEfSw+947EPsRRYoGogL18WyuhN8DqZV9es6UFt/nEnrN1Jlw5ITq6GeJtVmf4r012mVFL1SIL/qxrgPGZ4nkoEGjgVU= X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA2PR10MB4714 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.987,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2023-11-09_17,2023-11-09_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 spamscore=0 malwarescore=0 suspectscore=0 adultscore=0 bulkscore=0 mlxlogscore=778 phishscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311060000 definitions=main-2311090177 X-Proofpoint-ORIG-GUID: IfaEsNjxlfv4QvmhFHKMNxINEZNzgazp X-Proofpoint-GUID: IfaEsNjxlfv4QvmhFHKMNxINEZNzgazp Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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