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Tsirkin" , mtosatti@redhat.com Subject: Re: [PATCH v4 5/8] hpet: make main counter read lock-less Message-ID: <20250825171009.0651153f@fedora> In-Reply-To: References: <20250814160600.2327672-1-imammedo@redhat.com> <20250814160600.2327672-6-imammedo@redhat.com> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.49; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=imammedo@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, T_SPF_TEMPERROR=0.01 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, 25 Aug 2025 22:55:43 +0800 Zhao Liu wrote: > On Thu, Aug 14, 2025 at 06:05:57PM +0200, Igor Mammedov wrote: > > Date: Thu, 14 Aug 2025 18:05:57 +0200 > > From: Igor Mammedov > > Subject: [PATCH v4 5/8] hpet: make main counter read lock-less > > > > Make access to main HPET counter lock-less. > > > > In unlikely event of an update in progress, readers will busy wait > > untill update is finished. > > > > As result micro benchmark of concurrent reading of HPET counter > > with large number of vCPU shows over 80% better (less) latency. > > > > Signed-off-by: Igor Mammedov > > Reviewed-by: Peter Xu > > --- > > v3: > > * make reader busy wait during update and reuse existing seqlock API > > Peter Xu > > --- > > hw/timer/hpet.c | 26 ++++++++++++++++++++------ > > 1 file changed, 20 insertions(+), 6 deletions(-) > > ... > > > - QEMU_LOCK_GUARD(&s->lock); > > if (addr == HPET_COUNTER) { > > - if (hpet_enabled(s)) { > > - cur_tick = hpet_get_ticks(s); > > - } else { > > - cur_tick = s->hpet_counter; > > - } > > + unsigned version; > > + > > + /* > > + * Write update is rare, so busywait here is unlikely to happen > > + */ > > + do { > > + version = seqlock_read_begin(&s->state_version); > > + if (unlikely(!hpet_enabled(s))) { > > is there any particular consideration for rearranging the order of the > conditional branches here (and not directly using likely(hpet_enable()))? not really, I suppose it should be the same either way. > > > + cur_tick = s->hpet_counter; > > + } else { > > + cur_tick = hpet_get_ticks(s); > > + } > > + } while (seqlock_read_retry(&s->state_version, version)); > > trace_hpet_ram_read_reading_counter(addr & 4, cur_tick); > > return cur_tick >> shift; > > } > > Nice imprvoment! > > Reviewed-by: Zhao Liu > thanks!