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Goncalves" , Andreas Ziegler , Felix Moessbauer , Florian Bezdeka Subject: Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth controller Message-ID: <20250415062320.bMvqQsQ4@linutronix.de> References: <3f7b7ce1-6dd4-4a4e-9789-4c0cbde057bd@siemens.com> <20250409121314.GA632990@bytedance> <3d13e35a-51bb-4057-8923-ebb280793351@siemens.com> <20250414150513.klU46xkw@linutronix.de> <438a5250-30c0-4820-9084-cdef50f67865@siemens.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <438a5250-30c0-4820-9084-cdef50f67865@siemens.com> On 2025-04-15 07:35:50 [+0200], Jan Kiszka wrote: > > On RT the read_lock() in the timer block, the write blocks, too. So > > every blocker on the lock is scheduled out until the reader is gone. On > > top of that, the reader gets RCU boosted with FIFO-1 by default to get > > out. > > There is no boosting of the active readers on RT as there is no > information recorded about who is currently holding a read lock. This is > the whole point why rwlocks are hairy with RT, I thought. Kind of, yes. PREEMPT_RT has by default RCU boosting enabled with SCHED_FIFO 1. If you acquire a readlock you start a RCU section. If you get stuck in a RCU section for too long then this boosting will take effect by making the task, within the RCU section, the owner of the boost-lock and the boosting task will try to acquire it. This is used to get SCHED_OTHER tasks out of the RCU section. But if a SCHED_FIFO task is on the CPU then this boosting will have to no effect because the scheduler will not switch to a task with lower priority. > Jan Sebastian