Messages in this thread Patch in this message | | | Date | Wed, 14 Jul 2021 17:42:06 -0300 | From | Marcelo Tosatti <> | Subject | [patch 1/5] sched: isolation: introduce quiesce_on_exit_to_usermode isolcpu flags |
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Add a new isolcpus flag "quiesce_on_exit_to_usermode" to enable quiescing of deferred actions on return to userspace.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Index: linux-2.6-vmstat-update/include/linux/sched/isolation.h =================================================================== --- linux-2.6-vmstat-update.orig/include/linux/sched/isolation.h +++ linux-2.6-vmstat-update/include/linux/sched/isolation.h @@ -15,6 +15,7 @@ enum hk_flags { HK_FLAG_WQ = (1 << 6), HK_FLAG_MANAGED_IRQ = (1 << 7), HK_FLAG_KTHREAD = (1 << 8), + HK_FLAG_QUIESCE_URET = (1 << 9), }; #ifdef CONFIG_CPU_ISOLATION Index: linux-2.6-vmstat-update/kernel/sched/isolation.c =================================================================== --- linux-2.6-vmstat-update.orig/kernel/sched/isolation.c +++ linux-2.6-vmstat-update/kernel/sched/isolation.c @@ -173,6 +173,12 @@ static int __init housekeeping_isolcpus_ continue; } + if (!strncmp(str, "quiesce_on_exit_to_usermode,", 28)) { + str += 28; + flags |= HK_FLAG_QUIESCE_URET; + continue; + } + /* * Skip unknown sub-parameter and validate that it is not * containing an invalid character. Index: linux-2.6-vmstat-update/Documentation/admin-guide/kernel-parameters.txt =================================================================== --- linux-2.6-vmstat-update.orig/Documentation/admin-guide/kernel-parameters.txt +++ linux-2.6-vmstat-update/Documentation/admin-guide/kernel-parameters.txt @@ -2124,6 +2124,43 @@ The format of <cpu-list> is described above. + quiesce_on_exit_to_usermode + + This flag allows userspace to take preventive measures to + avoid deferred actions and create a OS noise free environment for + the application, by quiescing such activities on + return from syscalls (that is, perform the necessary + background work on return to userspace, rather than allowing + it to happen when userspace is executing, in the form of + an interruption to the application). + + There might be a performance degradation from using this, + on systemcall heavy workloads, for the isolated CPUs. + This option is intended to be used by specialized workloads. + + It should be deprecated in favour of a prctl() interface + to enable this mode (which allows the quiescing to take + place only on select sections of userspace execution, namely + the latency sensitive loops). + + Note: one of the preventive measures this option + enables is the following. + + Page counters are maintained in per-CPU counters to + improve performance. When a CPU modifies a page counter, + this modification is kept in the per-CPU counter. + Certain activities require a global count, which + involves requesting each CPU to flush its local counters + to the global VM counters. + This flush is implemented via a workqueue item, which + requires scheduling the workqueue task on isolated CPUs. + + To avoid this interruption, quiesce_on_exit_to_usermode + syncs the page counters on each return from system calls. + To ensure the application returns to userspace + with no modified per-CPU counters, its necessary to + use mlockall() in addition to this isolcpus flag. + iucv= [HW,NET] ivrs_ioapic [HW,X86-64]
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