linux-kernel - [patch 1/5] sched: isolation: introduce quiesce_on_exit_to

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Date:   Fri, 09 Jul 2021 14:37:27 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Christoph Lameter <cl@...ux.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Nitesh Lal <nilal@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolas Saenz <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
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@...hat.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]