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Date: Wed, 16 Dec 2020 17:41:30 +0000 From: Valentin Schneider <valentin.schneider@....com> To: Reinette Chatre <reinette.chatre@...el.com> Cc: tglx@...utronix.de, fenghua.yu@...el.com, bp@...en8.de, tony.luck@...el.com, kuo-lang.tseng@...el.com, shakeelb@...gle.com, mingo@...hat.com, babu.moger@....com, james.morse@....com, hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org Subject: Re: [PATCH 2/3] x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to resource group On 14/12/20 18:41, Reinette Chatre wrote: >>> - return ret; >>> + >>> + /* >>> + * By now, the task's closid and rmid are set. If the task is current >>> + * on a CPU, the PQR_ASSOC MSR needs to be updated to make the resource >>> + * group go into effect. If the task is not current, the MSR will be >>> + * updated when the task is scheduled in. >>> + */ >>> + update_task_closid_rmid(tsk); >> >> We need the above writes to be compile-ordered before the IPI is sent. >> There *is* a preempt_disable() down in smp_call_function_single() that >> gives us the required barrier(), can we deem that sufficient or would we >> want one before update_task_closid_rmid() for the sake of clarity? >> > > Apologies, it is not clear to me why the preempt_disable() would be > insufficient. If it is not then there may be a few other areas (where > resctrl calls smp_call_function_xxx()) that needs to be re-evaluated. So that's part paranoia and part nonsense from my end - the contents of smp_call() shouldn't matter here. If we distill the code to: tsk->closid = x; if (task_curr(tsk)) smp_call(...); It is somewhat far fetched, but AFAICT this can be compiled as: if (task_curr(tsk)) tsk->closid = x; smp_call(...); else tsk->closid = x; IOW, there could be a sequence where the closid write is ordered *after* the task_curr() read. With tsk->closid = x; barrier(); if (task_curr(tsk)) smp_call(...); that explicitely cannot happen. > > Thank you very much > > Reinette
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