From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v3 2/5] util: introduce threaded workqueue Date: Tue, 27 Nov 2018 16:29:05 +0800 Message-ID: References: <20181126184919.GA6688@flamenco> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, mst@redhat.com, mtosatti@redhat.com, Xiao Guangrong , dgilbert@redhat.com, peterx@redhat.com, qemu-devel@nongnu.org, quintela@redhat.com, wei.w.wang@intel.com, jiang.biao2@zte.com.cn, pbonzini@redhat.com To: "Emilio G. Cota" Return-path: In-Reply-To: <20181126184919.GA6688@flamenco> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel2=m.gmane.org@nongnu.org Sender: "Qemu-devel" List-Id: kvm.vger.kernel.org On 11/27/18 2:49 AM, Emilio G. Cota wrote: > On Mon, Nov 26, 2018 at 16:06:37 +0800, Xiao Guangrong wrote: >>>> + /* after the user fills the request, the bit is flipped. */ >>>> + uint64_t request_fill_bitmap QEMU_ALIGNED(SMP_CACHE_BYTES); >>>> + /* after handles the request, the thread flips the bit. */ >>>> + uint64_t request_done_bitmap QEMU_ALIGNED(SMP_CACHE_BYTES); >>> >>> Use DECLARE_BITMAP, otherwise you'll get type errors as David >>> pointed out. >> >> If we do it, the field becomes a pointer... that complicates the >> thing. > > Not necessarily, see below. > > On Mon, Nov 26, 2018 at 16:18:24 +0800, Xiao Guangrong wrote: >> On 11/24/18 8:17 AM, Emilio G. Cota wrote: >>> On Thu, Nov 22, 2018 at 15:20:25 +0800, guangrong.xiao@gmail.com wrote: >>>> +static uint64_t get_free_request_bitmap(Threads *threads, ThreadLocal *thread) >>>> +{ >>>> + uint64_t request_fill_bitmap, request_done_bitmap, result_bitmap; >>>> + >>>> + request_fill_bitmap = atomic_rcu_read(&thread->request_fill_bitmap); >>>> + request_done_bitmap = atomic_rcu_read(&thread->request_done_bitmap); >>>> + bitmap_xor(&result_bitmap, &request_fill_bitmap, &request_done_bitmap, >>>> + threads->thread_requests_nr); >>> >>> This is not wrong, but it's a big ugly. Instead, I would: >>> >>> - Introduce bitmap_xor_atomic in a previous patch >>> - Use bitmap_xor_atomic here, getting rid of the rcu reads >> >> Hmm, however, we do not need atomic xor operation here... that should be slower than >> just two READ_ONCE calls. > > If you use DECLARE_BITMAP, you get an in-place array. On a 64-bit > host, that'd be > unsigned long foo[1]; /* [2] on 32-bit */ > > Then again on 64-bit hosts, bitmap_xor_atomic would reduce > to 2 atomic reads: > > static inline void bitmap_xor_atomic(unsigned long *dst, > const unsigned long *src1, const unsigned long *src2, long nbits) > { > if (small_nbits(nbits)) { > *dst = atomic_read(src1) ^ atomic_read(&src2); > } else { > slow_bitmap_xor_atomic(dst, src1, src2, nbits); We needn't do inplace xor operation. i.e, we just fetch the bitmaps to the local variables do xor locally. So we need additional complicity to handle the case that is !small_nbits(nbits) ... but it is really not a big deal as you said, it just couple of codes. However, use u64 for the purpose that only 64 indexes are allowed is more straightforward and can be naturally understood. :)