Re: [PATCH] Reduce TLB flushing during process migration

Martin Hicks
2004-02-17T20:06:07Z

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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@colorfullife.com> 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@wildopensource.com 613-266-2296

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# 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@tomahawk.engr.sgi.com 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;
}