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SubjectRe: [regression] cpuidle_get_cpu_driver livelocks idle system
On 12/17/2012 08:36 PM, Russ Anderson wrote:
> The 3.7 kernel grinds to a halt on boot of a system with
> 2048 cpus. NMI showed most of the cpus in
> _raw_spin_lock in cpuidle_get_cpu_driver(). (backtrace below)
>
> A quick look at cpuidle_get_cpu_driver() shows the hot lock.
>
> In drivers/cpuidle/driver.c:
> --------------------------------------------------------
> /**
> * cpuidle_get_cpu_driver - return the driver tied with a cpu
> */
> struct cpuidle_driver *cpuidle_get_cpu_driver(struct cpuidle_device *dev)
> {
> struct cpuidle_driver *drv;
>
> if (!dev)
> return NULL;
>
> spin_lock(&cpuidle_driver_lock);
> drv = __cpuidle_get_cpu_driver(dev->cpu);
> spin_unlock(&cpuidle_driver_lock);
>
> return drv;
> }
> --------------------------------------------------------

Hi Russ,

thanks for investigating the problem. You are right, there is a
bottleneck here.

Regarding how is used the cpuidle code, I think it is safe to remove the
locks.



> This change was added in on Nov 14th, 2012.
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=bf4d1b5ddb78f86078ac6ae0415802d5f0c68f92
>
> The patch says it adds support for cpus with different characteristics,
> but adds a big global lock. The comment claims "no impact for the other
> platforms if the option is disabled", which leads me to believe the
> spin_lock was added inadvertently. CPU_IDLE_MULTIPLE_DRIVERS is off
> in my config file.
>
> linux$ grep CPU_IDLE_MULTIPLE_DRIVERS .config
> # CONFIG_CPU_IDLE_MULTIPLE_DRIVERS is not set
>
> As more cpus become idle, more cpus fight over the lock until
> the system livelocks on the crushing weight of idle.
>
> The fix may be to move the spin_lock into __cpuidle_get_cpu_driver,
> which has different versions for CONFIG_CPU_IDLE_MULTIPLE_DRIVERS,
> to avoid impacting the disabled case, or get rid of the spin_lock
> all together.
>
>
> --------------------------------------------------------
> == UV NMI process trace cpu 12: ==
> CPU 12
> Pid: 0, comm: swapper/12 Tainted: G O 3.7.0.rja-sgi+ #38
> RIP: 0010:[<ffffffff81614e45>] [<ffffffff81614e45>] _raw_spin_lock+0x25/0x30
> [...]
> Call Trace:
> [<ffffffff814c891c>] cpuidle_get_cpu_driver+0x1c/0x30
> [<ffffffff814c871d>] cpuidle_idle_call+0x7d/0x1b0
> [<ffffffff8101d08d>] cpu_idle+0xdd/0x130
> [<ffffffff8160a3ea>] start_secondary+0xc6/0xcc
> --------------------------------------------------------
>


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