Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 17 Jun 2019 17:19:02 -0700 | Subject | Re: [PATCH v4 3/5] x86/umwait: Add sysfs interface to control umwait C0.2 state |
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On Mon, Jun 17, 2019 at 5:09 PM Fenghua Yu <fenghua.yu@intel.com> wrote: > > On Mon, Jun 17, 2019 at 04:41:38PM -0700, Andy Lutomirski wrote: > > On Mon, Jun 17, 2019 at 4:20 PM Fenghua Yu <fenghua.yu@intel.com> wrote: > > > > > > On Mon, Jun 17, 2019 at 04:02:50PM -0700, Andy Lutomirski wrote: > > > > On Mon, Jun 17, 2019 at 1:36 PM Fenghua Yu <fenghua.yu@intel.com> wrote: > > > > > > > > > > On Mon, Jun 10, 2019 at 06:41:31AM -0700, Andy Lutomirski wrote: > > > > > > > > > > > > > > > > > > > On Jun 9, 2019, at 11:02 PM, Fenghua Yu <fenghua.yu@intel.com> wrote: > > > > > > > > > > > > > >> On Sun, Jun 09, 2019 at 09:24:18PM -0700, Andy Lutomirski wrote: > > > > > > >>> On Sun, Jun 9, 2019 at 9:02 PM Fenghua Yu <fenghua.yu@intel.com> wrote: > > > > > > >>> > > > > > > >>>> On Sat, Jun 08, 2019 at 03:50:32PM -0700, Andy Lutomirski wrote: > > > > > > >>>>> On Fri, Jun 7, 2019 at 3:10 PM Fenghua Yu <fenghua.yu@intel.com> wrote: > > > > > > >>>>> > > > > > > >>>>> C0.2 state in umwait and tpause instructions can be enabled or disabled > > > > > > >>>>> on a processor through IA32_UMWAIT_CONTROL MSR register. > > > > > > >>>>> > > > > > > >>>>> By default, C0.2 is enabled and the user wait instructions result in > > > > > > >>>>> lower power consumption with slower wakeup time. > > > > > > >>>>> > > > > > > >>>>> But in real time systems which require faster wakeup time although power > > > > > > >>>>> savings could be smaller, the administrator needs to disable C0.2 and all > > > > > > >>>>> C0.2 requests from user applications revert to C0.1. > > > > > > >>>>> > > > > > > >>>>> A sysfs interface "/sys/devices/system/cpu/umwait_control/enable_c02" is > > > > > > >>>>> created to allow the administrator to control C0.2 state during run time. > > > > > > >>>> > > > > > > >>>> This looks better than the previous version. I think the locking is > > > > > > >>>> still rather confused. You have a mutex that you hold while changing > > > > > > >>>> the value, which is entirely reasonable. But, of the code paths that > > > > > > >>>> write the MSR, only one takes the mutex. > > > > > > >>>> > > > > > > >>>> I think you should consider making a function that just does: > > > > > > >>>> > > > > > > >>>> wrmsr(MSR_IA32_UMWAIT_CONTROL, READ_ONCE(umwait_control_cached), 0); > > > > > > >>>> > > > > > > >>>> and using it in all the places that update the MSR. The only thing > > > > > > >>>> that should need the lock is the sysfs code to avoid accidentally > > > > > > >>>> corrupting the value, but that code should also use WRITE_ONCE to do > > > > > > >>>> its update. > > > > > > >>> > > > > > > >>> Based on the comment, the illustrative CPU online and enable_c02 store > > > > > > >>> functions would be: > > > > > > >>> > > > > > > >>> umwait_cpu_online() > > > > > > >>> { > > > > > > >>> wrmsr(MSR_IA32_UMWAIT_CONTROL, READ_ONCE(umwait_control_cached), 0); > > > > > > >>> return 0; > > > > > > >>> } > > > > > > >>> > > > > > > >>> enable_c02_store() > > > > > > >>> { > > > > > > >>> mutex_lock(&umwait_lock); > > > > > > >>> umwait_control_c02 = (u32)!c02_enabled; > > > > > > >>> WRITE_ONCE(umwait_control_cached, 2 | get_umwait_control_max_time()); > > > > > > >>> on_each_cpu(umwait_control_msr_update, NULL, 1); > > > > > > >>> mutex_unlock(&umwait_lock); > > > > > > >>> } > > > > > > >>> > > > > > > >>> Then suppose umwait_control_cached = 100000 initially and only CPU0 is > > > > > > >>> running. Admin change bit 0 in MSR from 0 to 1 to disable C0.2 and is > > > > > > >>> onlining CPU1 in the same time: > > > > > > >>> > > > > > > >>> 1. On CPU1, read umwait_control_cached to eax as 100000 in > > > > > > >>> umwait_cpu_online() > > > > > > >>> 2. On CPU0, write 100001 to umwait_control_cached in enable_c02_store() > > > > > > >>> 3. On CPU1, wrmsr with eax=100000 in umwaint_cpu_online() > > > > > > >>> 4. On CPU0, wrmsr with 100001 in enabled_c02_store() > > > > > > >>> > > > > > > >>> The result is CPU0 and CPU1 have different MSR values. > > > > > > >> > > > > > > >> Yes, but only transiently, because you didn't finish your example. > > > > > > >> > > > > > > >> Step 5: enable_c02_store() does on_each_cpu(), and CPU 1 gets updated. > > > > > > > > > > > > > > There is no sync on wrmsr on CPU0 and CPU1. > > > > > > > > > > > > What do you mean by sync? > > > > > > > > > > > > > So a better sequence to > > > > > > > describe the problem is changing the order of wrmsr: > > > > > > > > > > > > > > 1. On CPU1, read umwait_control_cached to eax as 100000 in > > > > > > > umwait_cpu_online() > > > > > > > 2. On CPU0, write 100001 to umwait_control_cached in enable_c02_store() > > > > > > > 3. On CPU0, wrmsr with 100001 in on_each_cpu() in enabled_c02_store() > > > > > > > 4. On CPU1, wrmsr with eax=100000 in umwaint_cpu_online() > > > > > > > > > > > > > > So CPU1 and CPU0 have different MSR values. This won't be transient. > > > > > > > > > > > > You are still ignoring the wrmsr on CPU1 due to on_each_cpu(). > > > > > > > > > > > > > > > > Initially umwait_control_cached is 100000 and CPU0 is online while CPU1 > > > > > is going to be online: > > > > > > > > > > 1. On CPU1, cpu_online_mask=0x3 in start_secondary() > > > > > 2. On CPU1, read umwait_control_cached to eax as 100000 in umwait_cpu_online() > > > > > 3. On CPU0, write 100001 to umwait_control_cached in enable_c02_store() > > > > > 4. On CPU0, execute one_each_cpu() in enabled_c02_store(): > > > > > wrmsr with 100001 on CPU0 > > > > > wrmsr with 100001 on CPU1 > > > > > 5. On CPU1, wrmsr with eax=100000 in umwaint_cpu_online() > > > > > > > > > > So the MSR is 100000 on CPU1 and 100001 on CPU0. The MSRs are different on > > > > > the CPUs. > > > > > > > > > > Is this a right sequence to demonstrate locking issue without the mutex > > > > > locking? > > > > > > > > > > > > > Fair enough. I would fix it differently, though: > > > > > > > > static void update_this_cpu_umwait_msr(void) > > > > { > > > > WARN_ON_ONCE(!irqs_disabled()); /* or local_irq_save() */ > > > > > > > > /* We need to prevent umwait_control from being changed *and* > > > > completing its WRMSR between our read and our WRMSR. By turning IRQs > > > > off here, we ensure that no sysfs write happens on this CPU and we > > > > also make sure that any concurrent sysfs write from a different CPU > > > > will not finish updating us via IPI until we're done. */ > > > > wrmsrl(MSR_..., READ_ONCE(umwait_control), 0); > > > > } > > > > > > If no other objections, then I will keep the current mutex lock/unlock to > > > protect wrmsr and the umwait_control_cached variable. > > > > > > > I don't think that's sufficient. In your current code, you hold the > > mutex in some places and not in others, and there's no explanation. > > The mutex is used in sysfs writing and cpu online. > > But it's not used in syscore resume because only BP is running syscore > resume. > > > And I think you're relying on the IRQs-off protection in at least one > > code path already, so you're not gaining any simplicity. > > I don't rely on IRQs-off protection. I only use mutex to protect.
You're relying on being single-threaded in umwait_syscore_resume(). Do you actually know that's safe? You say it's because you're single threaded, but what if you were suspended in the middle of a sysfs operation? I think it's fine, but it needs an argument along the lines of the argument for why the irqs disabled case is okay.
> > > At the very > > least, you need to add some extensive comments everywhere if you want > > to keep the mutex, > > I have comment on why no need for mutex protection in syscore resume. But > I can add more comments on the locking. > > > but I think it's simpler and clearer if you just > > use the same logic everywhere, for example, as I proposed above. > > But using irqs_disabled() before wrmsr() and READ_ONCE/WRITE_ONCE for > umwait_control_cached alone are not sufficient. The mutex is still needed > to protect sysfs writing, is that right? Without mutex, one_each_cpu() > can write different values on CPUs, right?
Yes, you probably need a mutex to prevent two sysfs writers from clobbering each other.
> > If irqs disabling, READ_ONCE/WRITE_ONCE, and mutex are all used to protect, > isn't that more complex than just using mutex?
But you're already using a mutex and a comment. And you're hoping that the syscore resume callback reads something sensible despite the lack of READ_ONCE / WRITE_ONCE. The compiler is unlikely to butcher this too badly, but still.
--Andy
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