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Date: Thu, 17 Aug 2017 15:29:16 +0800
From: Yang Zhang <yang.zhang.wz@...il.com>
To: "Michael S. Tsirkin" <mst@...hat.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 2017/8/16 12:04, Michael S. Tsirkin wrote:
> 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.
I am not sure mwait can solve this problem. But yes, if you have any
prototype patch, i can do a test. Also, i am working on next version
with better approach. I will post it ASAP.
>
>
>> 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
--
Yang
Alibaba Cloud Computing
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