Re: [PATCH] Reduce TLB flushing during process migration

[Martin Hicks]

On Tue, Feb 17, 2004 at 10:08:52AM -0800, David S. Miller wrote:
> On Tue, 17 Feb 2004 17:22:14 +0100
> Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:
> 
> > >+		 * we want a new context here. This eliminates TLB
> > >+		 * flushes on the cpus where the process executed prior to
> > >+		 * the migration.
> > >+		 */
> > >+		flush_tlb_mm(current->mm);
>  ...
> > I think flush_tlb_mm() is the wrong function - e.g. for i386, it's a 
> > wasted flush, because i386 disconnects previous cpus from the tlb flush 
> > automatically.
> > And it's always the wrong thing if you've migrated one thread of a task 
> > that runs on multiple cpus. I think you need a new hook.
> 
> Yes, you're probably right.  Just name it tlb_migrate_prepare(mm) or
> something like that.
> 
> I think most if not all non-x86 platforms will define this straight
> to flush_tlb_mm().

Here's an updated patch that creates the new wrapper.  I only changed
ia64 to call flush_tlb_mm() because I'm a little wary about assuming
that all non-x86 arches want this.

Boot tested on ia64, compile tested on x86.

mh

-- 
Martin Hicks                Wild Open Source Inc.
mort@xxxxxxxxxxxxxxxxxx     613-266-2296
# This is a BitKeeper generated patch for the following project:
# Project Name: Linux kernel tree
# This patch format is intended for GNU patch command version 2.5 or higher.
# This patch includes the following deltas:
#	           ChangeSet	1.1680  -> 1.1681 
#	include/asm-generic/tlb.h	1.20    -> 1.21   
#	include/asm-ia64/tlb.h	1.18    -> 1.19   
#	      kernel/sched.c	1.240   -> 1.241  
#
# The following is the BitKeeper ChangeSet Log
# --------------------------------------------
# 04/02/17	mort@xxxxxxxxxxxxxxxxxxxxx	1.1681
# Add tlb_migrate_prepare().  Most arches will define this to tlb_flush_mm().
# x86 is an exception.  For now make the default to do nothing.  Let each
# arch define tlb_migrate_prepare appropriately.
# --------------------------------------------
#
diff -Nru a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
--- a/include/asm-generic/tlb.h	Tue Feb 17 11:31:56 2004
+++ b/include/asm-generic/tlb.h	Tue Feb 17 11:31:56 2004
@@ -146,4 +146,6 @@
 		__pmd_free_tlb(tlb, pmdp);			\
 	} while (0)
 
+#define tlb_migrate_prepare(mm)
+
 #endif /* _ASM_GENERIC__TLB_H */
diff -Nru a/include/asm-ia64/tlb.h b/include/asm-ia64/tlb.h
--- a/include/asm-ia64/tlb.h	Tue Feb 17 11:31:56 2004
+++ b/include/asm-ia64/tlb.h	Tue Feb 17 11:31:56 2004
@@ -210,6 +210,8 @@
 	tlb->end_addr = address + PAGE_SIZE;
 }
 
+#define tlb_migrate_prepare(mm) flush_tlb_mm(mm)
+
 #define tlb_start_vma(tlb, vma)			do { } while (0)
 #define tlb_end_vma(tlb, vma)			do { } while (0)
 
diff -Nru a/kernel/sched.c b/kernel/sched.c
--- a/kernel/sched.c	Tue Feb 17 11:31:56 2004
+++ b/kernel/sched.c	Tue Feb 17 11:31:56 2004
@@ -25,6 +25,7 @@
 #include <linux/highmem.h>
 #include <linux/smp_lock.h>
 #include <asm/mmu_context.h>
+#include <asm/tlb.h>
 #include <linux/interrupt.h>
 #include <linux/completion.h>
 #include <linux/kernel_stat.h>
@@ -1135,6 +1136,14 @@
 		task_rq_unlock(rq, &flags);
 		wake_up_process(rq->migration_thread);
 		wait_for_completion(&req.done);
+
+		/*
+		 * we want a new context here. This eliminates TLB
+		 * flushes on the cpus where the process executed prior to
+		 * the migration.
+		 */
+		tlb_migrate_prepare(current->mm);
+
 		return;
 	}
 out: