Messages in this thread | | | Date | Tue, 16 Jul 2013 14:26:52 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH 7/8] cpufreq: Preserve policy structure across suspend/resume |
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On 07/16/2013 11:45 AM, Viresh Kumar wrote: > On 15 July 2013 15:35, Srivatsa S. Bhat > <srivatsa.bhat@linux.vnet.ibm.com> wrote: >> Actually even I was wondering about this while writing the patch and >> I even tested shutdown after multiple suspend/resume cycles, to verify that >> the refcount is messed up. But surprisingly, things worked just fine. >> >> Logically there should've been a refcount mismatch and things should have >> failed, but everything worked fine during my tests. Apart from suspend/resume >> and shutdown tests, I even tried mixing a few regular CPU hotplug operations >> (echo 0/1 to sysfs online files), but nothing stood out. >> >> Sorry, I forgot to document this in the patch. Either the patch is wrong >> or something else is silently fixing this up. Not sure what is the exact >> situation. > > To understand it I actually applied your patches to get better view of the code. > (Haven't tested it though).. And found that your code is doing the right thing > and we shouldn't get a mismatch.. This is the sequence of events I can draw: > > - __cpu_add_dev() for first cpu. sets the refcount to 'x', where x are > the no. of > cpus in its clock domain. > - _cpu_add_dev() for other cpus: doesn't change anything in refcount > > - Suspend: > - cpu_remove_dev() for all cpus, due to frozen flag we don't touch the value > of count > - Resume: > - cpu_add_dev() for all cpus, due to frozen flag we don't touch the > value of count. >
Actually this one is tricky (I took a look again). So we have this code in the beginning of _cpufreq_add_dev():
1008 #ifdef CONFIG_SMP 1009 /* check whether a different CPU already registered this 1010 * CPU because it is in the same boat. */ 1011 policy = cpufreq_cpu_get(cpu); 1012 if (unlikely(policy)) { 1013 cpufreq_cpu_put(policy); 1014 return 0; 1015 }
The _get() is not controlled by the frozen flag, but it still doesn't take a refcount because of a subtle reason: per_cpu(cpufreq_cpu_data, cpu) was set to NULL in __cpufreq_remove_dev() and the memory was saved away in fallback storage. So, when __cpufreq_cpu_get() executes, it sees:
204 /* get the CPU */ 205 data = per_cpu(cpufreq_cpu_data, cpu); 206 207 if (!data) 208 goto err_out_put_module;
Thus, since data is NULL, cpufreq_cpu_get() won't take a refcount and will return silently.
Further down in __cpufreq_add_dev(), we restore the original memory, using the frozen flag:
1037 if (frozen) 1038 /* Restore the saved policy when doing light-weight init */ 1039 policy = cpufreq_policy_restore(cpu); 1040 else 1041 policy = cpufreq_policy_alloc();
So that is how we manage to fool cpufreq_cpu_get() into not taking a fresh refcount while resuming :) > And so things work as expected. That's why your code isn't breaking anything I > believe. >
Thanks a lot for the code inspection and your detailed analysis!
> But can no. of cpus change inbetween suspend/resume? Then count would be > tricky as we are using the same policy structure. >
No, number of CPUs won't change in between suspend/resume. Even if somebody tried that, that would be an eccentric case and we won't handle that. Besides, *many more* things will break than just cpufreq, if somebody actually tries that out!
Regards, Srivatsa S. Bhat
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