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Date: Wed, 25 Nov 2020 23:23:57 +0000 From: Valentin Schneider <valentin.schneider@....com> To: Reinette Chatre <reinette.chatre@...el.com> Cc: linux-kernel@...r.kernel.org, x86@...nel.org, Fenghua Yu <fenghua.yu@...el.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, James Morse <James.Morse@....com> Subject: Re: [PATCH v2 0/3] x86/intel_rdt: task_work vs task_struct rmid/closid write race On 25/11/20 19:06, Reinette Chatre wrote: > Hi Valentin, > > On 11/25/2020 10:39 AM, Valentin Schneider wrote: >> The (default) TWA_RESUME ensures the targeted (userspace) task gets kicked >> if it is currently running, and doesn't perturb any CPU otherwise; >> see set_notify_resume() + exit_to_user_mode_loop() (or do_notify_resume() >> on arm64) > > I missed that, thanks. The first issue is thus not a problem. Thank you > very much for clearing this up. Queueing work for tasks that are not > running remains unnecessary and simplifying this with a targeted > smp_call_function addresses that (while also taking care of the other > issues with using the queued work). > Right. >>> In the new solution, after updating closid/rmid in the task_struct, the >>> CPU register is updated via smp_call_function_single() on a CPU the task >>> is running. Nothing is done for tasks not running, next time they are >>> scheduled in the CPU's register will be updated to reflect the task's >>> closid/rmid. Moving to the smp_call_function_xxx() API would also bring >>> this update in line with how other register updates are already done in >>> resctrl. >>> >>>> Kernel threads however are a prickly matter because they quite explicitly >>>> don't have this return to userspace - they only run their task_work >>>> callbacks on exit. So we currently have to wait for those kthreads to go >>>> through a context switch to update the relevant register, but I don't >>>> see any other alternative that wouldn't involve interrupting every other >>>> CPU (the kthread could move between us triggering some remote work and its >>>> previous CPU receiving the IPI). >>> >>> This seems ok? In the new solution the closid/rmid would be updated in >>> task_struct and a smp_call_function_single() attempted on the CPU where >>> the kthread is running. If the kthread is no longer running at the time >>> the function is called the CPU register will not be changed. >> >> Right, if the update happens before triggering the remote work then that >> should all work. I was stuck thinking about keeping the update contained >> within the remote work itself to prevent any other races (i.e. patch 3). > > Are you saying that the task_struct update as well as register update > should both be done in the remote work? I think I may be > misunderstanding though. It would simplify the concurrency aspect - if the {closid, rmid} update is always done on the targeted task' context, then there can be no races between an update (write) and a context switch (read). Sadly I don't see a nice way to do this for kthreads, so I think it'll have to be update + smp_call. > Currently, with your entire series applied, the > update to task_struct is done before the remote work is queued that only > changes the register. The new solution would also first update the > task_struct and then the remote work (this time with smp_call_function) > will just update the register. > > From what I understand your work in patch 3 would continue to be > welcome with the new solution that will also update the task_struct and > then trigger the remote work to just update the register. > That's how I see it as well ATM. >> Anywho, that's enough speculation from me, I'll just sit tight and see what >> comes next! >> > > Reinette > >>> I assume >>> the kthread move would include a context switch that would result in the >>> register change (__switch_to()->resctrl_sched_in()) for the kthread to >>> run with its new closid/rmid after the move. >>> > > > Reinette
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