From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9A14412FB1B for ; Fri, 27 Dec 2024 15:55:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735314904; cv=none; b=XrWIfKaB8kSIoSYhpw90QTe1uLN/6GVvmZpFXVn5WJSnqICCAqKED64BoFmv9u9WuUZScR03khIzZ1eH/POeVbLUUQeoO/6RVVRul+4jFGAcTX1VoZ8X+t3kuGJjtpK13Etgz5v0u559Y7UMu1qrwU9U6poGJmSf79mcLw66SZk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735314904; c=relaxed/simple; bh=HnKiEwm2THC3dybJ3DMwhpRF0xhDOToiw8YqPmch4jg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Y35MIQy62D/DheAqKpRcX3IhDWKwB+xQ6RO0MRDAJkYTQCIPfSF8PNtzE0fTRxgK3N5v2NjacEW3KkmJGN5mW+GbUzbdcuvuBJwc8vR1a7HovaxwrwQR+QA3LkGs8hnMWVeQBpruqXX9n2ZoraGXCG2BWv5WtaHvC9IXzLiCqxk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=V6+Hjgig; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="V6+Hjgig" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1735314901; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=XtsRlXvexnQ2mwHiMNXBb80D5vWX4b75zLU2RpTUBs0=; b=V6+HjgigHe1p2zmr/nHd49zBluJ34N853Q57JanfPPxPLAiLbi8jG/fvAucds/JhMymYPY cJun8/rwO4na0VQSY9tlezOC/YZrS7JVtsSxXaC03Ki0SrMwCFVY/oJ7YHk6qGx1XUO1gH DefkjyCToUgcA2jeRn7xLk3JENS8V58= Received: from mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-224-njBQ0bz5MXmvwv_Cy1ykVg-1; Fri, 27 Dec 2024 10:54:58 -0500 X-MC-Unique: njBQ0bz5MXmvwv_Cy1ykVg-1 X-Mimecast-MFC-AGG-ID: njBQ0bz5MXmvwv_Cy1ykVg Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4E3AB195608E; Fri, 27 Dec 2024 15:54:56 +0000 (UTC) Received: from dhcp-27-174.brq.redhat.com (unknown [10.45.224.44]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id 3064A1956053; Fri, 27 Dec 2024 15:54:53 +0000 (UTC) Received: by dhcp-27-174.brq.redhat.com (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Fri, 27 Dec 2024 16:54:31 +0100 (CET) Date: Fri, 27 Dec 2024 16:54:28 +0100 From: Oleg Nesterov To: Linus Torvalds Cc: WangYuli , linux-fsdevel , Linux Kernel Mailing List , Christian Brauner Subject: Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write Message-ID: <20241227155428.GA15300@redhat.com> References: <75B06EE0B67747ED+20241225094202.597305-1-wangyuli@uniontech.com> <20241226201158.GB11118@redhat.com> <20241226205746.GC11118@redhat.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241226205746.GC11118@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On 12/26, Oleg Nesterov wrote: > > On 12/26, Linus Torvalds wrote: > > > > So the optimization may be valid > > I don't think so, see my initial reply. > > unlike wait_event(), __pollwait() + the head/tail checks in pipe_poll() > doesn't have the necessary barriers (at least in theory) afaics. Between > add_wait_queue()->list_add() and LOAD(head/tail). Hmm... Even if we add the wq_has_sleeper() check, the "wake up" logic would be still suboptimal. Lets forget this patch for the moment. Consider int main(void) { int fd[2], cnt; char c; pipe(fd); if (!fork()) { // wait until the parent blocks in pipe_write() -> // wait_event_interruptible_exclusive(pipe->wr_wait, pipe_writable(pipe)); sleep(1); for (cnt = 0; cnt < 4096; ++cnt) read(fd[0], &c, 1); return 0; } // parent for (;;) write(fd[1], &c, 1); } In this case the child will wakeup the parent 4095 times for no reason, pipe_writable() == !pipe_pull() will still be true until the last read(fd[0], &c, 1) does if (!buf->len) tail = pipe_update_tail(pipe, buf, tail); and after that the parent can write the next char. Or did I completely misread this code?? Oleg.