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Date: Sun, 21 Apr 2024 15:27:37 -0300 From: "Guilherme G. Piccoli" <gpiccoli@...lia.com> To: Maksim Davydov <davydov-max@...dex-team.ru> Cc: den-plotnikov@...dex-team.ru, x86@...nel.org, linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de Subject: Re: [PATCH] x86/split_lock: fix delayed detection enabling On 19/04/2024 08:26, Maksim Davydov wrote: > > [...] > Some information that should be taken into account: > * sld_update_msr() enables/disables SLD on both CPUs on the same core > * schedule_delayed_work_on() internally checks WORK_STRUCT_PENDING_BIT. > If a work has the 'pending' status, then schedule_delayed_work_on() > will return an error code and, most importantly, the work will not > be placed in the workqueue. > > Let's say we have a multicore system on which split_lock_mitigate=0 and > a multithreaded application is running that calls splitlock in multiple > threads. Due to the fact that sld_update_msr() affects the entire core > (both CPUs), we will consider 2 CPUs from different cores. Let the 2 > threads of this application schedule to CPU0 (core 0) and to CPU 2 > (core 1), then: > > | || | > | CPU 0 (core 0) || CPU 2 (core 1) | > |_________________________________||___________________________________| > | || | > | 1) SPLIT LOCK occured || | > | || | > | 2) split_lock_warn() || | > | || | > | 3) sysctl_sld_mitigate == 0 || | > | (work = &sl_reenable) || | > | || | > | 4) schedule_delayed_work_on() || | > | (reenable will be called || | > | after 2 jiffies on CPU 0) || | > | || | > | 5) disable SLD for core 0 || | > | || | > | ------------------------- || | > | || | > | || 6) SPLIT LOCK occured | > | || | > | || 7) split_lock_warn() | > | || | > | || 8) sysctl_sld_mitigate == 0 | > | || (work = &sl_reenable, | > | || the same address as in 3) ) | > | || | > | 2 jiffies || 9) schedule_delayed_work_on() | > | || fials because the work is in | > > | || the pending state since 4). | > | || The work wasn't placed to the | > | || workqueue. reenable won't be | > | || called on CPU 2 | > | || | > | || 10) disable SLD for core 0 | > | || | > | || From now on SLD will | > | || never be reenabled on core 1 | > | || | > | ------------------------- || | > | || | > | 11) enable SLD for core 0 by || | > | __split_lock_reenable || | > | || | > > > If the application threads can be scheduled to all processor cores, > then over time there will be only one core left, on which SLD will be > enabled and split lock will be able to be detected; and on all other > cores SLD will be disabled all the time. > Most likely, this bug has not been noticed for so long because > sysctl_sld_mitigate default value is 1, and in this case a semaphore > is used that does not allow 2 different cores to have SLD disabled at > the same time, that is, strictly only one work is placed in the > workqueue. > Hi Maksim, this is awesome! Thanks a lot for the diagram, super clear now. Well, I think you nailed it and we should get the patch merged, right? I'm not sure if the diagram should be included or not in the commit message - it's good but big, maybe include a lore archive mentioning the diagram in a V2? Cheers, Guilherme
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