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Thu, 19 Oct 2017 01:03:21 -0700 From: Dongli Zhang To: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org Date: Thu, 19 Oct 2017 16:02:59 +0800 Message-Id: <1508400179-10204-1-git-send-email-dongli.zhang@oracle.com> X-Mailer: git-send-email 2.7.4 X-Source-IP: userv0021.oracle.com [156.151.31.71] Cc: jgross@suse.com, boris.ostrovsky@oracle.com, joao.m.martins@oracle.com Subject: [Xen-devel] [PATCH 1/1] xen/time: do not decrease steal time after live migration on xen X-BeenThere: xen-devel@lists.xen.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , MIME-Version: 1.0 Errors-To: xen-devel-bounces@lists.xen.org Sender: "Xen-devel" X-Virus-Scanned: ClamAV using ClamSMTP 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 The code in this patch is borrowed from do_stolen_accounting() which has already been removed from linux source code since commit ecb23dc6f2ef ("xen: add steal_clock support on x86"). The core idea of both do_stolen_accounting() and this patch is to avoid accounting new steal clock if it is smaller than previous old steal clock. 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 --- drivers/xen/time.c | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/drivers/xen/time.c b/drivers/xen/time.c index ac5f23f..2b3a996 100644 --- a/drivers/xen/time.c +++ b/drivers/xen/time.c @@ -19,6 +19,8 @@ /* runstate info updated by Xen */ static DEFINE_PER_CPU(struct vcpu_runstate_info, xen_runstate); +static DEFINE_PER_CPU(u64, xen_old_steal); + /* return an consistent snapshot of 64-bit time/counter value */ static u64 get64(const u64 *p) { @@ -83,9 +85,20 @@ bool xen_vcpu_stolen(int vcpu) u64 xen_steal_clock(int cpu) { struct vcpu_runstate_info state; + u64 xen_new_steal; + s64 steal_delta; xen_get_runstate_snapshot_cpu(&state, cpu); - return state.time[RUNSTATE_runnable] + state.time[RUNSTATE_offline]; + xen_new_steal = state.time[RUNSTATE_runnable] + + state.time[RUNSTATE_offline]; + steal_delta = xen_new_steal - per_cpu(xen_old_steal, cpu); + + if (steal_delta < 0) + xen_new_steal = per_cpu(xen_old_steal, cpu); + else + per_cpu(xen_old_steal, cpu) = xen_new_steal; + + return xen_new_steal; } void xen_setup_runstate_info(int cpu)