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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id v1-20020a05620a0f0100b006bb8b5b79efsm19281127qkl.129.2022.10.05.12.48.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 05 Oct 2022 12:48:32 -0700 (PDT) Date: Wed, 5 Oct 2022 15:48:30 -0400 From: Peter Xu To: "Dr. David Alan Gilbert" Cc: qemu-devel@nongnu.org, Manish Mishra , Juan Quintela , ani@anisinha.ca, Leonardo Bras Soares Passos , "Daniel P . Berrange" Subject: Re: [PATCH 05/14] migration: Yield bitmap_mutex properly when sending/sleeping Message-ID: References: <20220920225106.48451-1-peterx@redhat.com> <20220920225210.48732-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Oct 05, 2022 at 09:40:53AM -0400, Peter Xu wrote: > On Wed, Oct 05, 2022 at 12:18:00PM +0100, Dr. David Alan Gilbert wrote: > > * Peter Xu (peterx@redhat.com) wrote: > > > On Tue, Oct 04, 2022 at 02:55:10PM +0100, Dr. David Alan Gilbert wrote: > > > > * Peter Xu (peterx@redhat.com) wrote: > > > > > Don't take the bitmap mutex when sending pages, or when being throttled by > > > > > migration_rate_limit() (which is a bit tricky to call it here in ram code, > > > > > but seems still helpful). > > > > > > > > > > It prepares for the possibility of concurrently sending pages in >1 threads > > > > > using the function ram_save_host_page() because all threads may need the > > > > > bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of > > > > > qemu_sem_wait() blocking for one thread will not block the other from > > > > > progressing. > > > > > > > > > > Signed-off-by: Peter Xu > > > > > > > > I generally dont like taking locks conditionally; but this kind of looks > > > > OK; I think it needs a big comment on the start of the function saying > > > > that it's called and left with the lock held but that it might drop it > > > > temporarily. > > > > > > Right, the code is slightly hard to read, I just didn't yet see a good and > > > easy solution for it yet. It's just that we may still want to keep the > > > lock as long as possible for precopy in one shot. > > > > > > > > > > > > --- > > > > > migration/ram.c | 42 +++++++++++++++++++++++++++++++----------- > > > > > 1 file changed, 31 insertions(+), 11 deletions(-) > > > > > > > > > > diff --git a/migration/ram.c b/migration/ram.c > > > > > index 8303252b6d..6e7de6087a 100644 > > > > > --- a/migration/ram.c > > > > > +++ b/migration/ram.c > > > > > @@ -2463,6 +2463,7 @@ static void postcopy_preempt_reset_channel(RAMState *rs) > > > > > */ > > > > > static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > > > { > > > > > + bool page_dirty, release_lock = postcopy_preempt_active(); > > > > > > > > Could you rename that to something like 'drop_lock' - you are taking the > > > > lock at the end even when you have 'release_lock' set - which is a bit > > > > strange naming. > > > > > > Is there any difference on "drop" or "release"? I'll change the name > > > anyway since I definitely trust you on any English comments, but please > > > still let me know - I love to learn more on those! :) > > > > I'm not sure 'drop' is much better either; I was struggling to find a > > good nam. > > I can also call it "preempt_enabled". > > Actually I can directly replace it with calling postcopy_preempt_active() > always but I just want to make it crystal clear that the value is not > changing and lock & unlock are always paired - in our case I think it is > not changing, but the var helps to be 100% sure there'll be no possible bug > on e.g. deadlock caused by state changing. > > > > > > > > > > > > int tmppages, pages = 0; > > > > > size_t pagesize_bits = > > > > > qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS; > > > > > @@ -2486,22 +2487,41 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss) > > > > > break; > > > > > } > > > > > > > > > > + page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page); > > > > > + /* > > > > > + * Properly yield the lock only in postcopy preempt mode because > > > > > + * both migration thread and rp-return thread can operate on the > > > > > + * bitmaps. > > > > > + */ > > > > > + if (release_lock) { > > > > > + qemu_mutex_unlock(&rs->bitmap_mutex); > > > > > + } > > > > > > > > Shouldn't the unlock/lock move inside the 'if (page_dirty) {' ? > > > > > > I think we can move into it, but it may not be as optimal as keeping it > > > as-is. > > > > > > Consider a case where we've got the bitmap with continous zero bits. > > > During postcopy, the migration thread could be spinning here with the lock > > > held even if it doesn't send a thing. It could still block the other > > > return path thread on sending urgent pages which may be outside the zero > > > zones. > > > > OK, that reason needs commenting then - you're going to do a lot of > > release/take pairs in that case which is going to show up as very hot; > > so hmm, if ti was just for that type of 'yield' behaviour you wouldn't > > normally do it for each bit. > > Hold on.. I think my assumption won't easily trigger, because at the end of > the loop we'll try to look for the next "dirty" page. So continuously > clean pages are unlikely, or I even think it's impossible because we're > holding the mutex during scanning and clear-dirty, so no one will be able > to flip the bit. > > So yeah I think it's okay to move it into "page_dirty", but since we'll > mostly always go into dirty maybe it's just that it won't help a lot > either, because it'll be mostly the same as keeping it outside? IOW, maybe I should drop page_dirty directly and replace it with a check, failing migration if migration_bitmap_clear_dirty() returned false? -- Peter Xu