Messages in this thread | | | Subject | Re: [PATCH v3 1/7] arm64: add support for the AMU extension v1 | From | Suzuki K Poulose <> | Date | Wed, 12 Feb 2020 19:24:13 +0000 |
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Hi Ionela,
On 02/12/2020 06:20 PM, Ionela Voinescu wrote: > Hi Suzuki, > > On Wednesday 12 Feb 2020 at 16:20:56 (+0000), Suzuki Kuruppassery Poulose wrote: >>>>> +static bool has_amu(const struct arm64_cpu_capabilities *cap, >>>>> + int __unused) >>>>> +{ >>>>> + /* >>>>> + * The AMU extension is a non-conflicting feature: the kernel can >>>>> + * safely run a mix of CPUs with and without support for the >>>>> + * activity monitors extension. Therefore, if not disabled through >>>>> + * the kernel command line early parameter, enable the capability >>>>> + * to allow any late CPU to use the feature. >>>>> + * >>>>> + * With this feature enabled, the cpu_enable function will be called >>>>> + * for all CPUs that match the criteria, including secondary and >>>>> + * hotplugged, marking this feature as present on that respective CPU. >>>>> + * The enable function will also print a detection message. >>>>> + */ >>>>> + >>>>> + if (!disable_amu && !zalloc_cpumask_var(&amu_cpus, GFP_KERNEL)) { >>>> >>>> This looks problematic. Don't we end up in allocating the memory during >>>> "each CPU" check and thus leaking memory ? Do we really need to allocate >>>> this dynamically ? >>>> >>> >>> Yes, it does make some assumptions. Given that the AMU capability is >>> a WEAK_LOCAL_CPU_FEATURE I relied on the match function being called >>> only once, when the return value is true. If the return value is false, >> >> That is not correct. A WEAK_LOCAL_CPU_FEATURE is still SCOPE_LOCAL_CPU, >> implies it is run on all the booting CPUs (including the hotplugged >> ones). The WEAK is there to imply that its "permitted" or "optional" >> for a hotplugged CPU. So, eventually you will re-allocate this variable >> every single time a CPU turns up, where you could also loose the current >> state. >> > >>> which will result in it being called multiple times, it's either because >>> disable_amu == false, or it has become false due to a previous failed >>> allocation, in which case a new allocation will not be attempted. > > First of all, I agree with you that this should be corrected. > > But for completion (and my education) I retraced my steps in regards > to my assumption above. While cpu_enable is called for all CPUs - boot, > secondary, hotplugged, the matches function (in this case has_amu) is > not always called for all CPUs, and that's where the confusion came > from. > > Looking over the update_cpu_capabilities function, that's called from > both setup_boot_cpu_capabilities and check_local_cpu_capabilities > (secondary CPUs) for SCOPE_LOCAL_CPU: > > ----- > static void update_cpu_capabilities(u16 scope_mask) > { > int i; > const struct arm64_cpu_capabilities *caps; > > scope_mask &= ARM64_CPUCAP_SCOPE_MASK; > for (i = 0; i < ARM64_NCAPS; i++) { > caps = cpu_hwcaps_ptrs[i]; > if (!caps || !(caps->type & scope_mask) || > cpus_have_cap(caps->capability) || > !caps->matches(caps, cpucap_default_scope(caps))) > continue; > > --> The matches function is only called if !cpus_have_cap
Agreed. Your analysis is correct. This was done as a micro optimization(!) as it is pointless to check if something should be set, when it is already set.
> > > if (caps->desc) > pr_info("detected: %s\n", caps->desc); > cpus_set_cap(caps->capability); > > --> If matches returns true we mark it as present in cpu_hwcaps. > > if ((scope_mask & SCOPE_BOOT_CPU) && (caps->type & SCOPE_BOOT_CPU)) > set_bit(caps->capability, boot_capabilities); > } > } > --- > > Therefore caps->matches (in this case has_amu) will only be called as > long as it returns false. This is where my assumption above came from. > Also, this is the reason it was working nicely in my testing, as I did > not test hotplug this time. > > Where the has_amu code breaks is when we end up calling > verify_local_cpu_capabilities instead of update_cpu_capabilities after > sys_caps_initialised, which will happen for hotplugged CPUs. > In that case we call caps->matches for all CPUs. Also, if anyone in the > future ends up calling this_cpu_has_cap for the AMU capability.
True.
> > I will fix this.
Cheers Suzuki
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