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Date: Wed, 16 Aug 2017 07:04:53 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: root <yang.zhang.wz@...il.com>
Cc: tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
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Subject: Re: [PATCH 1/2] x86/idle: add halt poll for halt idle
On Thu, Jun 22, 2017 at 11:22:13AM +0000, root wrote:
> From: Yang Zhang <yang.zhang.wz@...il.com>
>
> This patch introduce a new mechanism to poll for a while before
> entering idle state.
>
> David has a topic in KVM forum to describe the problem on current KVM VM
> when running some message passing workload in KVM forum. Also, there
> are some work to improve the performance in KVM, like halt polling in KVM.
> But we still has 4 MSR wirtes and HLT vmexit when going into halt idle
> which introduce lot of latency.
>
> Halt polling in KVM provide the capbility to not schedule out VCPU when
> it is the only task in this pCPU. Unlike it, this patch will let VCPU polls
> for a while if there is no work inside VCPU to elimiate heavy vmexit during
> in/out idle. The potential impact is it will cost more CPU cycle since we
> are doing polling and may impact other task which waiting on the same
> physical CPU in host.
I wonder whether you considered doing this in an idle driver.
I have a prototype patch combining this with mwait within guest -
I can post it if you are interested.
> Here is the data i get when running benchmark contextswitch
> (https://github.com/tsuna/contextswitch)
>
> before patch:
> 2000000 process context switches in 4822613801ns (2411.3ns/ctxsw)
>
> after patch:
> 2000000 process context switches in 3584098241ns (1792.0ns/ctxsw)
>
> Signed-off-by: Yang Zhang <yang.zhang.wz@...il.com>
> ---
> Documentation/sysctl/kernel.txt | 10 ++++++++++
> arch/x86/kernel/process.c | 21 +++++++++++++++++++++
> include/linux/kernel.h | 3 +++
> kernel/sched/idle.c | 3 +++
> kernel/sysctl.c | 9 +++++++++
> 5 files changed, 46 insertions(+)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index bac23c1..4e71bfe 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -63,6 +63,7 @@ show up in /proc/sys/kernel:
> - perf_event_max_stack
> - perf_event_max_contexts_per_stack
> - pid_max
> +- poll_threshold_ns [ X86 only ]
> - powersave-nap [ PPC only ]
> - printk
> - printk_delay
> @@ -702,6 +703,15 @@ kernel tries to allocate a number starting from this one.
>
> ==============================================================
>
> +poll_threshold_ns: (X86 only)
> +
> +This parameter used to control the max wait time to poll before going
> +into real idle state. By default, the values is 0 means don't poll.
> +It is recommended to change the value to non-zero if running latency-bound
> +workloads in VM.
> +
> +==============================================================
> +
> powersave-nap: (PPC only)
>
> If set, Linux-PPC will use the 'nap' mode of powersaving,
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 0bb8842..6361783 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -39,6 +39,10 @@
> #include <asm/desc.h>
> #include <asm/prctl.h>
>
> +#ifdef CONFIG_HYPERVISOR_GUEST
> +unsigned long poll_threshold_ns;
> +#endif
> +
> /*
> * per-CPU TSS segments. Threads are completely 'soft' on Linux,
> * no more per-task TSS's. The TSS size is kept cacheline-aligned
> @@ -313,6 +317,23 @@ static inline void play_dead(void)
> }
> #endif
>
> +#ifdef CONFIG_HYPERVISOR_GUEST
> +void arch_cpu_idle_poll(void)
> +{
> + ktime_t start, cur, stop;
> +
> + if (poll_threshold_ns) {
> + start = cur = ktime_get();
> + stop = ktime_add_ns(ktime_get(), poll_threshold_ns);
> + do {
> + if (need_resched())
> + break;
> + cur = ktime_get();
> + } while (ktime_before(cur, stop));
> + }
> +}
> +#endif
> +
> void arch_cpu_idle_enter(void)
> {
> tsc_verify_tsc_adjust(false);
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 13bc08a..04cf774 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -460,6 +460,9 @@ extern __scanf(2, 0)
> extern int sysctl_panic_on_stackoverflow;
>
> extern bool crash_kexec_post_notifiers;
> +#ifdef CONFIG_HYPERVISOR_GUEST
> +extern unsigned long poll_threshold_ns;
> +#endif
>
> /*
> * panic_cpu is used for synchronizing panic() and crash_kexec() execution. It
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 2a25a9e..e789f99 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -74,6 +74,7 @@ static noinline int __cpuidle cpu_idle_poll(void)
> }
>
> /* Weak implementations for optional arch specific functions */
> +void __weak arch_cpu_idle_poll(void) { }
> void __weak arch_cpu_idle_prepare(void) { }
> void __weak arch_cpu_idle_enter(void) { }
> void __weak arch_cpu_idle_exit(void) { }
> @@ -219,6 +220,8 @@ static void do_idle(void)
> */
>
> __current_set_polling();
> + arch_cpu_idle_poll();
> +
> tick_nohz_idle_enter();
>
> while (!need_resched()) {
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 4dfba1a..9174d57 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1203,6 +1203,15 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
> .extra2 = &one,
> },
> #endif
> +#ifdef CONFIG_HYPERVISOR_GUEST
> + {
> + .procname = "halt_poll_threshold",
> + .data = &poll_threshold_ns,
> + .maxlen = sizeof(unsigned long),
> + .mode = 0644,
> + .proc_handler = proc_dointvec,
> + },
> +#endif
> { }
> };
>
> --
> 1.8.3.1
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