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Subject[patch 1/5] sched: isolation: introduce quiesce_on_exit_to_usermode isolcpu flags
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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