Commit 4be1b297 authored Feb 12, 2015 by Cyril Bur Committed by Linus Torvalds Feb 12, 2015

powerpc: add running_clock for powerpc to prevent spurious softlockup warnings



On POWER8 virtualised kernels the VTB register can be read to have a view
of time that only increases while the guest is running.  This will prevent
guests from seeing time jump if a guest is paused for significant amounts
of time.

On POWER7 and below virtualised kernels stolen time is subtracted from
local_clock as a best effort approximation.  This will not eliminate
spurious warnings in the case of a suspended guest but may reduce the
occurance in the case of softlockups due to host over commit.

Bare metal kernels should avoid reading the VTB as KVM does not restore
sane values when not executing, the approxmation is fine as host kernels
won't observe any stolen time.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Andrew Jones <drjones@redhat.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Cc: chai wen <chaiw.fnst@cn.fujitsu.com>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Aaron Tomlin <atomlin@redhat.com>
Cc: Ben Zhang <benzh@chromium.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 545a2bf7

arch/powerpc/kernel/time.c

+32 −0

Original line number	Diff line number	Diff line
		@@ -621,6 +621,38 @@ unsigned long long sched_clock(void)
		return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;
		}


		#ifdef CONFIG_PPC_PSERIES

		/*
		* Running clock - attempts to give a view of time passing for a virtualised
		* kernels.
		* Uses the VTB register if available otherwise a next best guess.
		*/
		unsigned long long running_clock(void)
		{
		/*
		* Don't read the VTB as a host since KVM does not switch in host
		* timebase into the VTB when it takes a guest off the CPU, reading the
		* VTB would result in reading 'last switched out' guest VTB.
		*
		* Host kernels are often compiled with CONFIG_PPC_PSERIES checked, it
		* would be unsafe to rely only on the #ifdef above.
		*/
		if (firmware_has_feature(FW_FEATURE_LPAR) &&
		cpu_has_feature(CPU_FTR_ARCH_207S))
		return mulhdu(get_vtb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;

		/*
		* This is a next best approximation without a VTB.
		* On a host which is running bare metal there should never be any stolen
		* time and on a host which doesn't do any virtualisation TB should equal
		* VTB so it makes no difference anyway.
		*/
		return local_clock() - cputime_to_nsecs(kcpustat_this_cpu->cpustat[CPUTIME_STEAL]);
		}
		#endif

		static int __init get_freq(char name, int cells, unsigned long val)
		{
		struct device_node *cpu;

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