After guest live migration on xen, steal time in /proc/stat (cpustat[CPUTIME_STEAL]) might decrease because steal returned by xen_steal_lock() might be less than this_rq()->prev_steal_time which is derived from previous return value of xen_steal_clock().
For instance, steal time of each vcpu is 335 before live migration. cpu 198 0 368 200064 1962 0 0 1340 0 0 cpu0 38 0 81 50063 492 0 0 335 0 0 cpu1 65 0 97 49763 634 0 0 335 0 0 cpu2 38 0 81 50098 462 0 0 335 0 0 cpu3 56 0 107 50138 374 0 0 335 0 0 After live migration, steal time is reduced to 312. cpu 200 0 370 200330 1971 0 0 1248 0 0 cpu0 38 0 82 50123 500 0 0 312 0 0 cpu1 65 0 97 49832 634 0 0 312 0 0 cpu2 39 0 82 50167 462 0 0 312 0 0 cpu3 56 0 107 50207 374 0 0 312 0 0 Since runstate times are cumulative and cleared during xen live migration by xen hypervisor, the idea of this patch is to accumulate runstate times to global percpu variables before live migration suspend. Once guest VM is resumed, xen_get_runstate_snapshot_cpu() would always return the sum of new runstate times and previously accumulated times stored in global percpu variables. Similar and more severe issue would impact prior linux 4.8-4.10 as discussed by Michael Las at https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest, which would overflow steal time and lead to 100% st usage in top command for linux 4.8-4.10. A backport of this patch would fix that issue. References: https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest Signed-off-by: Dongli Zhang <dongli.zh...@oracle.com> --- Changed since v1: * relocate modification to xen_get_runstate_snapshot_cpu Changed since v2: * accumulate runstate times before live migration Changed since v3: * do not accumulate times in the case of guest checkpointing --- drivers/xen/manage.c | 2 ++ drivers/xen/time.c | 83 ++++++++++++++++++++++++++++++++++++++++++-- include/xen/interface/vcpu.h | 2 ++ include/xen/xen-ops.h | 1 + 4 files changed, 86 insertions(+), 2 deletions(-) diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c index c425d03..3dc085d 100644 --- a/drivers/xen/manage.c +++ b/drivers/xen/manage.c @@ -72,6 +72,7 @@ static int xen_suspend(void *data) } gnttab_suspend(); + xen_accumulate_runstate_time(-1); xen_arch_pre_suspend(); /* @@ -84,6 +85,7 @@ static int xen_suspend(void *data) : 0); xen_arch_post_suspend(si->cancelled); + xen_accumulate_runstate_time(si->cancelled); gnttab_resume(); if (!si->cancelled) { diff --git a/drivers/xen/time.c b/drivers/xen/time.c index ac5f23f..18e2b76 100644 --- a/drivers/xen/time.c +++ b/drivers/xen/time.c @@ -19,6 +19,9 @@ /* runstate info updated by Xen */ static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate); +static DEFINE_PER_CPU(u64[RUNSTATE_max], old_runstate_time); +static u64 **runstate_time_delta; + /* return an consistent snapshot of 64-bit time/counter value */ static u64 get64(const u64 *p) { @@ -47,8 +50,8 @@ static u64 get64(const u64 *p) return ret; } -static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res, - unsigned int cpu) +static void xen_get_runstate_snapshot_cpu_delta( + struct vcpu_runstate_info *res, unsigned int cpu) { u64 state_time; struct vcpu_runstate_info *state; @@ -66,6 +69,82 @@ static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res, (state_time & XEN_RUNSTATE_UPDATE)); } +static void xen_get_runstate_snapshot_cpu(struct vcpu_runstate_info *res, + unsigned int cpu) +{ + int i; + + xen_get_runstate_snapshot_cpu_delta(res, cpu); + + for (i = 0; i < RUNSTATE_max; i++) + res->time[i] += per_cpu(old_runstate_time, cpu)[i]; +} + +void xen_accumulate_runstate_time(int action) +{ + struct vcpu_runstate_info state; + int cpu, i; + + switch (action) { + case -1: /* backup runstate time before suspend */ + WARN_ON_ONCE(unlikely(runstate_time_delta)); + + runstate_time_delta = kcalloc(num_possible_cpus(), + sizeof(*runstate_time_delta), + GFP_KERNEL); + if (unlikely(!runstate_time_delta)) { + pr_alert("%s: failed to allocate runstate_time_delta\n", + __func__); + return; + } + + for_each_possible_cpu(cpu) { + runstate_time_delta[cpu] = kmalloc_array(RUNSTATE_max, + sizeof(**runstate_time_delta), + GFP_KERNEL); + if (unlikely(!runstate_time_delta[cpu])) { + pr_alert("%s: failed to allocate runstate_time_delta[%d]\n", + __func__, cpu); + action = 0; + goto reclaim_mem; + } + + xen_get_runstate_snapshot_cpu_delta(&state, cpu); + memcpy(runstate_time_delta[cpu], + state.time, + RUNSTATE_max * sizeof(**runstate_time_delta)); + } + break; + + case 0: /* backup runstate time after resume */ + if (unlikely(!runstate_time_delta)) { + pr_alert("%s: cannot accumulate runstate time as runstate_time_delta is NULL\n", + __func__); + return; + } + + for_each_possible_cpu(cpu) { + for (i = 0; i < RUNSTATE_max; i++) + per_cpu(old_runstate_time, cpu)[i] += + runstate_time_delta[cpu][i]; + } + break; + + default: /* do not accumulate runstate time for checkpointing */ + break; + } + +reclaim_mem: + if (action != -1 && runstate_time_delta) { + for_each_possible_cpu(cpu) { + if (likely(runstate_time_delta[cpu])) + kfree(runstate_time_delta[cpu]); + } + kfree(runstate_time_delta); + runstate_time_delta = NULL; + } +} + /* * Runstate accounting */ diff --git a/include/xen/interface/vcpu.h b/include/xen/interface/vcpu.h index 98188c8..85e81ce 100644 --- a/include/xen/interface/vcpu.h +++ b/include/xen/interface/vcpu.h @@ -110,6 +110,8 @@ DEFINE_GUEST_HANDLE_STRUCT(vcpu_runstate_info); */ #define RUNSTATE_offline 3 +#define RUNSTATE_max 4 + /* * Register a shared memory area from which the guest may obtain its own * runstate information without needing to execute a hypercall. diff --git a/include/xen/xen-ops.h b/include/xen/xen-ops.h index 218e6aa..b1f9ae9 100644 --- a/include/xen/xen-ops.h +++ b/include/xen/xen-ops.h @@ -32,6 +32,7 @@ void xen_resume_notifier_unregister(struct notifier_block *nb); bool xen_vcpu_stolen(int vcpu); void xen_setup_runstate_info(int cpu); void xen_time_setup_guest(void); +void xen_accumulate_runstate_time(int action); void xen_get_runstate_snapshot(struct vcpu_runstate_info *res); u64 xen_steal_clock(int cpu); -- 2.7.4
