From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56464) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gRWNf-00071E-3X for qemu-devel@nongnu.org; Tue, 27 Nov 2018 00:57:56 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gRWNa-0007so-8c for qemu-devel@nongnu.org; Tue, 27 Nov 2018 00:57:55 -0500 Received: from mga07.intel.com ([134.134.136.100]:41194) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gRWNa-0007sX-0B for qemu-devel@nongnu.org; Tue, 27 Nov 2018 00:57:50 -0500 Message-ID: <5BFCDE07.20707@intel.com> Date: Tue, 27 Nov 2018 14:02:47 +0800 From: Wei Wang MIME-Version: 1.0 References: <1542276484-25508-1-git-send-email-wei.w.wang@intel.com> <1542276484-25508-4-git-send-email-wei.w.wang@intel.com> <20181127054056.GA3205@xz-x1> In-Reply-To: <20181127054056.GA3205@xz-x1> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v9 3/8] migration: use bitmap_mutex in migration_bitmap_clear_dirty List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Xu Cc: qemu-devel@nongnu.org, virtio-dev@lists.oasis-open.org, mst@redhat.com, quintela@redhat.com, dgilbert@redhat.com, pbonzini@redhat.com, liliang.opensource@gmail.com, nilal@redhat.com, riel@redhat.com On 11/27/2018 01:40 PM, Peter Xu wrote: > On Thu, Nov 15, 2018 at 06:07:59PM +0800, Wei Wang wrote: >> The bitmap mutex is used to synchronize threads to update the dirty >> bitmap and the migration_dirty_pages counter. For example, the free >> page optimization clears bits of free pages from the bitmap in an >> iothread context. This patch makes migration_bitmap_clear_dirty update >> the bitmap and counter under the mutex. >> >> Signed-off-by: Wei Wang >> CC: Dr. David Alan Gilbert >> CC: Juan Quintela >> CC: Michael S. Tsirkin >> CC: Peter Xu >> --- >> migration/ram.c | 3 +++ >> 1 file changed, 3 insertions(+) >> >> diff --git a/migration/ram.c b/migration/ram.c >> index 7e7deec..ef69dbe 100644 >> --- a/migration/ram.c >> +++ b/migration/ram.c >> @@ -1556,11 +1556,14 @@ static inline bool migration_bitmap_clear_dirty(RAMState *rs, >> { >> bool ret; >> >> + qemu_mutex_lock(&rs->bitmap_mutex); >> ret = test_and_clear_bit(page, rb->bmap); >> >> if (ret) { >> rs->migration_dirty_pages--; >> } >> + qemu_mutex_unlock(&rs->bitmap_mutex); >> + >> return ret; >> } > It seems fine to me, but have you thought about > test_and_clear_bit_atomic()? Note that we just had > test_and_set_bit_atomic() a few months ago. Thanks for sharing. I think we might also need to mutex migration_dirty_pages. > > And not related to this patch: I'm unclear on why we have had > bitmap_mutex before, since it seems unnecessary. OK. This is because with the optimization we have a thread which clears bits (of free pages) from the bitmap and updates migration_dirty_pages. So we need to synchronization between the migration thread and the optimization thread. Best, Wei