linux/arch/x86
Takuya Yoshikawa 60c34612b7 KVM: Switch to srcu-less get_dirty_log()
We have seen some problems of the current implementation of
get_dirty_log() which uses synchronize_srcu_expedited() for updating
dirty bitmaps; e.g. it is noticeable that this sometimes gives us ms
order of latency when we use VGA displays.

Furthermore the recent discussion on the following thread
    "srcu: Implement call_srcu()"
    http://lkml.org/lkml/2012/1/31/211
also motivated us to implement get_dirty_log() without SRCU.

This patch achieves this goal without sacrificing the performance of
both VGA and live migration: in practice the new code is much faster
than the old one unless we have too many dirty pages.

Implementation:

The key part of the implementation is the use of xchg() operation for
clearing dirty bits atomically.  Since this allows us to update only
BITS_PER_LONG pages at once, we need to iterate over the dirty bitmap
until every dirty bit is cleared again for the next call.

Although some people may worry about the problem of using the atomic
memory instruction many times to the concurrently accessible bitmap,
it is usually accessed with mmu_lock held and we rarely see concurrent
accesses: so what we need to care about is the pure xchg() overheads.

Another point to note is that we do not use for_each_set_bit() to check
which ones in each BITS_PER_LONG pages are actually dirty.  Instead we
simply use __ffs() in a loop.  This is much faster than repeatedly call
find_next_bit().

Performance:

The dirty-log-perf unit test showed nice improvements, some times faster
than before, except for some extreme cases; for such cases the speed of
getting dirty page information is much faster than we process it in the
userspace.

For real workloads, both VGA and live migration, we have observed pure
improvements: when the guest was reading a file during live migration,
we originally saw a few ms of latency, but with the new method the
latency was less than 200us.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>
2012-04-08 12:50:00 +03:00
..
boot x86, boot: Correct CFLAGS for hostprogs 2012-03-22 12:42:51 -07:00
configs x86/kconfig: Remove CONFIG_TR=y from the defconfigs 2012-03-24 08:18:03 +01:00
crypto Merge git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2012-03-22 20:19:30 -07:00
ia32 Merge branch 'x86-cleanups-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-29 18:21:35 -07:00
include/asm KVM: Avoid checking huge page mappings in get_dirty_log() 2012-04-08 12:49:58 +03:00
kernel kvmclock: remove unneeded EXPORT macro 2012-04-08 12:49:54 +03:00
kvm KVM: Switch to srcu-less get_dirty_log() 2012-04-08 12:50:00 +03:00
lguest lguest: Make sure interrupt is allocated ok by lguest_setup_irq 2012-01-12 15:44:47 +10:30
lib Merge branch 'x86-atomic-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-22 09:23:57 -07:00
math-emu x86: Rename trap_no to trap_nr in thread_struct 2012-03-13 06:24:09 +01:00
mm Merge branch 'x86-cleanups-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-29 18:21:35 -07:00
net bpf jit: Let the x86 jit handle negative offsets 2012-04-03 18:01:41 -04:00
oprofile x86: Factor out TIF_IA32 from 32-bit address space 2012-02-20 12:48:46 -08:00
pci Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jbarnes/pci 2012-03-23 14:02:12 -07:00
platform Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-29 14:28:26 -07:00
power x86, kvm: Call restore_sched_clock_state() only after %gs is initialized 2012-04-02 13:53:00 +02:00
syscalls Merge branch 'x86-x32-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-29 18:12:23 -07:00
tools x86/tools: Add decoded instruction dump mode 2011-12-05 14:53:23 +01:00
um Merge branch 'x86-x32-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-29 18:12:23 -07:00
vdso Merge branch 'x86-x32-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-29 18:12:23 -07:00
video x86: fix up files really needing to include module.h 2011-10-31 19:30:36 -04:00
xen Two fixes for regressions: 2012-04-06 17:54:53 -07:00
.gitignore x86/kprobes: Add arch/x86/tools/insn_sanity to .gitignore 2012-01-16 08:21:59 +01:00
Kbuild
Kconfig Merge branch 'x86-x32-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-03-29 18:12:23 -07:00
Kconfig.cpu x86: Tighten dependencies of CPU_SUP_*_32 2012-03-08 10:57:34 +01:00
Kconfig.debug Merge branch 'x86-platform-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-01-11 19:13:40 -08:00
Makefile x86: Use -mno-avx when available 2012-03-30 10:06:39 -07:00
Makefile_32.cpu
Makefile.um um: most of the SUBARCH uses can be killed 2012-03-25 00:29:56 +01:00