On 14/11/17 08:02, Quan Xu wrote: > > > On 2017/11/13 18:53, Juergen Gross wrote: >> On 13/11/17 11:06, Quan Xu wrote: >>> From: Quan Xu <quan.xu0@xxxxxxxxx> >>> >>> So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called >>> in idle path which will poll for a while before we enter the real idle >>> state. >>> >>> In virtualization, idle path includes several heavy operations >>> includes timer access(LAPIC timer or TSC deadline timer) which will >>> hurt performance especially for latency intensive workload like message >>> passing task. The cost is mainly from the vmexit which is a hardware >>> context switch between virtual machine and hypervisor. Our solution is >>> to poll for a while and do not enter real idle path if we can get the >>> schedule event during polling. >>> >>> Poll may cause the CPU waste so we adopt a smart polling mechanism to >>> reduce the useless poll. >>> >>> Signed-off-by: Yang Zhang <yang.zhang.wz@xxxxxxxxx> >>> Signed-off-by: Quan Xu <quan.xu0@xxxxxxxxx> >>> Cc: Juergen Gross <jgross@xxxxxxxx> >>> Cc: Alok Kataria <akataria@xxxxxxxxxx> >>> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx> >>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> >>> Cc: Ingo Molnar <mingo@xxxxxxxxxx> >>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> >>> Cc: x86@xxxxxxxxxx >>> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx >>> Cc: linux-kernel@xxxxxxxxxxxxxxx >>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx >> Hmm, is the idle entry path really so critical to performance that a new >> pvops function is necessary? > Juergen, Here is the data we get when running benchmark netperf: > 1. w/o patch and disable kvm dynamic poll (halt_poll_ns=0): > 29031.6 bit/s -- 76.1 %CPU > > 2. w/ patch and disable kvm dynamic poll (halt_poll_ns=0): > 35787.7 bit/s -- 129.4 %CPU > > 3. w/ kvm dynamic poll: > 35735.6 bit/s -- 200.0 %CPU > > 4. w/patch and w/ kvm dynamic poll: > 42225.3 bit/s -- 198.7 %CPU > > 5. idle=poll > 37081.7 bit/s -- 998.1 %CPU > > > > w/ this patch, we will improve performance by 23%.. even we could improve > performance by 45.4%, if we use w/patch and w/ kvm dynamic poll. also the > cost of CPU is much lower than 'idle=poll' case.. I don't question the general idea. I just think pvops isn't the best way to implement it. >> Wouldn't a function pointer, maybe guarded >> by a static key, be enough? A further advantage would be that this would >> work on other architectures, too. > > I assume this feature will be ported to other archs.. a new pvops makes > code > clean and easy to maintain. also I tried to add it into existed pvops, > but it > doesn't match. You are aware that pvops is x86 only? I really don't see the big difference in maintainability compared to the static key / function pointer variant: void (*guest_idle_poll_func)(void); struct static_key guest_idle_poll_key __read_mostly; static inline void guest_idle_poll(void) { if (static_key_false(&guest_idle_poll_key)) guest_idle_poll_func(); } And KVM would just need to set guest_idle_poll_func and enable the static key. Works on non-x86 architectures, too. Juergen -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html