Re: [PATCH] locking/osq: Drop the overload of osq lock

[xinhui]

On 2016å06æ27æ 14:45, Boqun Feng wrote:
> On Sun, Jun 26, 2016 at 02:59:26PM +0800, Boqun Feng wrote:
> [snip]
> > This should be:
> >
> > extern struct vcpu_preempt_ops vcpu_preempt_ops;
> >
> > And I tested this one along with modified version of Xinhui's patch.
> >
> > The test showed that even in a not over-committed system, the overhead
> > of mutex_lock() could go down from 2.58%(w/o patches) to 0.87%(w/
> > patches).
>
> Clearly, I made several more mistakes in the patch, and the vcpu
> preemption could never been activated, so please ignore this bench
> result.
>
> Here is a new version of vcpu preempt. Thoughts?
>
> Regards,
> Boqun
>
> ---------------->8
> virt: Introduce vcpu preemption detection functions
>
> Lock holder preemption is an serious problem in virtualized environment
> for locking. Different archs and hypervisors employ different ways to
> try to solve the problem in different locking mechanisms. And recently
> Pan Xinhui found a significant performance drop in osq_lock(), which
> could be solved by providing a mechanism to detect the preemption of
> vcpu.
>
> Therefore this patch introduces several vcpu preemption detection
> primitives, which allows locking primtives to save the time of
> unnecesarry spinning. These primitives act as an abstract layer between
> locking functions and arch or hypervisor related code.
>
> There are two sets of primitives:
>
> 1.	vcpu_preempt_count() and vcpu_has_preempted(), they must be used
> 	pairwisely in a same preempt disable critical section. And they
> 	could detect whether a vcpu preemption happens between them.
>
> 2.	vcpu_is_preempted(), used to check whether other cpu's vcpu is
> 	preempted.
>
> This patch also implements those primitives on pseries and wire them up.
>
>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---
>   arch/powerpc/platforms/pseries/Kconfig |  1 +
>   arch/powerpc/platforms/pseries/setup.c | 28 ++++++++++++
>   include/linux/vcpu_preempt.h           | 82 ++++++++++++++++++++++++++++++++++
>   kernel/Kconfig.preempt                 |  5 ++-
>   4 files changed, 115 insertions(+), 1 deletion(-)
>   create mode 100644 include/linux/vcpu_preempt.h
>
> diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
> index bec90fb30425..7d24c3e48878 100644
> --- a/arch/powerpc/platforms/pseries/Kconfig
> +++ b/arch/powerpc/platforms/pseries/Kconfig
> @@ -21,6 +21,7 @@ config PPC_PSERIES
>   	select HOTPLUG_CPU if SMP
>   	select ARCH_RANDOM
>   	select PPC_DOORBELL
> +	select HAS_VCPU_PREEMPTION_DETECTION
>   	default y
>
>   config PPC_SPLPAR
> diff --git a/arch/powerpc/platforms/pseries/setup.c b/arch/powerpc/platforms/pseries/setup.c
> index 9883bc7ea007..5d4aed54e039 100644
> --- a/arch/powerpc/platforms/pseries/setup.c
> +++ b/arch/powerpc/platforms/pseries/setup.c
> @@ -42,6 +42,7 @@
>   #include <linux/of.h>
>   #include <linux/of_pci.h>
>   #include <linux/kexec.h>
> +#include <linux/vcpu_preempt.h>
>
>   #include <asm/mmu.h>
>   #include <asm/processor.h>
> @@ -501,6 +502,31 @@ static void __init find_and_init_phbs(void)
>   	of_pci_check_probe_only();
>   }
>
> +struct vcpu_preempt_ops vcpu_preempt_ops = DEFAULT_VCPU_PREEMPT_OPS;
> +EXPORT_SYMBOL(vcpu_preempt_ops);
> +
> +static long pseries_vcpu_preempt_count(void)
> +{
> +	return be32_to_cpu(get_lppaca()->yield_count);
> +}
> +
> +static bool pseries_vcpu_is_preempted(int cpu)
> +{
> +	return !!(be32_to_cpu(lppaca_of(cpu).yield_count) & 1);
> +}
> +
> +static bool pseries_vcpu_has_preempted(long vpc)
> +{
> +	return pseries_vcpu_preempt_count() != vpc;
> +}
> +
> +static void __init pseries_setup_vcpu_preempt_ops(void)
> +{
> +	vcpu_preempt_ops.preempt_count = pseries_vcpu_preempt_count;
> +	vcpu_preempt_ops.is_preempted = pseries_vcpu_is_preempted;
> +	vcpu_preempt_ops.has_preempted = pseries_vcpu_has_preempted;
> +}
> +
>   static void __init pSeries_setup_arch(void)
>   {
>   	set_arch_panic_timeout(10, ARCH_PANIC_TIMEOUT);
> @@ -549,6 +575,8 @@ static void __init pSeries_setup_arch(void)
>   				"%ld\n", rc);
>   		}
>   	}
> +
> +	pseries_setup_vcpu_preempt_ops();
>   }
>
>   static int __init pSeries_init_panel(void)
> diff --git a/include/linux/vcpu_preempt.h b/include/linux/vcpu_preempt.h
> new file mode 100644
> index 000000000000..e76e054a7917
> --- /dev/null
> +++ b/include/linux/vcpu_preempt.h
> @@ -0,0 +1,82 @@
> +/*
> + * Primitives for checking the vcpu preemption from the guest.
> + */
> +
> +static long __vcpu_preempt_count(void)
> +{
> +	return 0;
> +}
> +
> +static bool __vcpu_has_preempted(long vpc)
> +{
> +	return false;
> +}
> +
> +static bool __vcpu_is_preempted(int cpu)
> +{
> +	return false;
> +}
> +
> +struct vcpu_preempt_ops {
> +	/*
> +	 * Get the current vcpu's "preempt count", which is going to use for
> +	 * checking whether the current vcpu has ever been preempted
> +	 */
> +	long (*preempt_count)(void);
> +
> +	/*
> +	 * Return whether a vcpu is preempted
> +	 */
> +	bool (*is_preempted)(int cpu);
> +
> +	/*
> +	 * Given a "vcpu preempt count", Return whether a vcpu preemption ever
> +	 * happened after the .preempt_count() was called.
> +	 */
> +	bool (*has_preempted)(long vpc);
> +};
> +
> +extern struct vcpu_preempt_ops vcpu_preempt_ops;
> +
> +/* Default boilerplate */
> +#define DEFAULT_VCPU_PREEMPT_OPS			\
> +	{						\
> +		.preempt_count = __vcpu_preempt_count,	\
> +		.is_preempted = __vcpu_is_preempted,	\
> +		.has_preempted = __vcpu_has_preempted	\
> +	}
> +
> +#ifdef CONFIG_HAS_VCPU_PREEMPTION_DETECTION
> +/*
> + * vcpu_preempt_count: Get the current cpu's "vcpu preempt count"(vpc).
> + *
> + * The vpc is used for checking whether the current vcpu has ever been
> + * preempted via vcpu_has_preempted().
> + *
> + * This function and vcpu_has_preepmted() should be called in the same
> + * preemption disabled critical section.
> + */
> +#define vcpu_preempt_count()	vcpu_preempt_ops.preempt_count()
> +
> +/*
> + * vcpu_is_preempted: Check whether @cpu's vcpu is preempted.
> + */
> +#define vcpu_is_preempted(cpu)	vcpu_preempt_ops.is_preempted(cpu)
> +
> +/*
> + * vcpu_has_preepmted: Check whether the current cpu's vcpu has ever been
> + * preempted.
> + *
> + * The checked duration is between the vcpu_preempt_count() which returns @vpc
> + * is called and this function called.
> + *
> + * This function and corresponding vcpu_preempt_count() should be in the same
> + * preemption disabled cirtial section.
> + */
> +#define vcpu_has_preempted(vpc)	vcpu_preempt_ops.has_preempted(vpc)
> +
> +#else /* CONFIG_HAS_VCPU_PREEMPTION_DETECTION */
> +#define vcpu_preempt_count() __vcpu_preempt_count()
> +#define vcpu_is_preempted(cpu) __vcpu_is_preempted(cpu)
> +#define vcpu_has_preempted(vpc) __vcpu_has_preempted(vpc)
> +#endif /* CONFIG_HAS_VCPU_PREEPMTION_DETECTION */

AND if possible, We can hide the detail about vcpu preempted check. IOW, caller better do not know vcpu_preempt_count() .

currently, users have to code like below.

long vpc = vcpu_preempt_count();
while()
	if (vcpu_has_preempted(vpc))
		break;
the vpc is long, this the implementation detail.



SO we can introduce :
vcpu_preempt_critical_start(void);
{
	/* record the yield count*/
	__this_cpu_write(vpc, vcpu_preempt_count());
}
vcpu_preempt_critical_end(void)
{
	// do nothing.
}

vcpu_has_preempted(void)
{
	vcpu_preempt_ops.has_preempted(__this_cpu_read(vpc));
}


then we can code like below

vcpu_preempt_critical_start()
while()
	if (vcpu_has_preempted())
		break;
vcpu_preempt_critical_end()

looks a little better?

thanks
xinhui

> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index 3f9c97419f02..cb88bc3a2fc6 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -55,4 +55,7 @@ config PREEMPT
>   endchoice
>
>   config PREEMPT_COUNT
> -       bool
> \ No newline at end of file
> +       bool
> +
> +config HAS_VCPU_PREEMPTION_DETECTION
> +       bool