From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=91.218.175.82 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793473121; b=D/wcHAUHysXRYl/gyZcWVz16Fdvot5bOcbYQvDCYG+K5qqE9LsIKZOZ4/PguRUnfa18Z GlFmmv/Ol1GH31x9ua/JXxzRvsAUQnOuOZ1sDkIVTQ8v04UZIl3OvmhGUqjNCXJp44T5q 7+u1pyiIZUg1GIsIsVJFe2NdWhCdl429anZKXcATi110fxuRQHFVkmw0L/XF+Fh/eZn8M ZIywpmnTCRardTmr+L5CgczCXdfBarbYns6ehgvz9mP1eMWTnaSGdu1ppYpM81/o0GBzm +TBctRyKdyWihPZBNa9kV79LjTbwFzQeMIqtbbx6mN3+D+SoToadRAx5zEyW2LjjPOQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793473121; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=WTXPO9m21ChhSJhJsMR1jm9N3k/S9PtVs0GDOD3AsIY=; b=Xi31KQacoJp2Pe9ogljeLt9exbNA+n9YRmKbu3mqUhbn9CmXyOU3H9LXywD/OI1rfc1q 0lTEBPXiNQh1ZUkRI8qoh4J+MWOUz8r1vo3eXIRhbuZfThYdTf6Hg7IORDt6HNzh/Qg1C bAFJzq39/UBVNplniX7lXroJ+jyEwivkNpAuHQzRFkY8jqN9mqDf3SAcL7VJeskbLNHUg qY9R2OuDUk4saEDTg/e3u0p707zVbNtJzFnWWjJDzHPbfZn7uoO1mG1GL7dEeGlv+2pTw xpPueV3PHu7F6l8CwAq/g/0Yg5w1ZlTfqIXNFlqhVsjFQtgE0kSqUMoUluZIXMYpSyg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; arc=none smtp.remote-ip=91.218.175.82 Received: from mta0.migadu.com (out-82.mta0.migadu.com [91.218.175.82]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 87B62606A320 for ; Sat, 31 Oct 2026 18:58:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 87B62606A320 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev X-Envelope-To: kvm@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=nhn60L6iDcU1tEzuh3dkIqEoHLNq1R18VibAiPoFZc4=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793473113; v=1; x=1794077913; b=FgLPquBtbge3POVC+2Dr8m+4QENK0dVztMZkqUikUBrZvJtE6H91VKKswJxIsBJL67pY6Jz9 6g8qzsLTfA9LttvrY33jPtsEM+SjPu1YN01qWSJr0tzgcny6phbIpfbk016/wYEPf6rRAq9FOJQ UmOda5mSHbWZxP2yJv5ZIimA= X-Envelope-To: kvm@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id d88e81d4557f9af7; Sat, 31 Oct 2026 18:58:33 +0000 X-Mizu-Trace-ID: d88e81d4557f9af7 X-Migadu-Flow: FLOW_OUT Date: Sat, 31 Oct 2026 11:58:31 -0700 From: Shakeel Butt To: "Michael S. Tsirkin" Cc: Jason Wang , Eugenio =?utf-8?B?UMOpcmV6?= , Meta kernel team , cgroups@vger.kernel.org, linux-mmc@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] vhost_task: do the return-to-userspace work between work items Message-ID: References: <20261031181004.4193627-1-shakeel.butt@linux.dev> <20261031141506-mutt-send-email-mst@kernel.org> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261031141506-mutt-send-email-mst@kernel.org> On Sat, Oct 31, 2026 at 02:15:40PM -0400, Michael S. Tsirkin wrote: > On Sat, Oct 31, 2026 at 11:10:04AM -0700, Shakeel Butt wrote: > > A vhost task is a user worker: a thread of a user process that runs only > > kernel code. It never returns to userspace, so it never runs the work > > that is queued for the way back. resume_user_mode_work() does that work > > when TIF_NOTIFY_RESUME is set. For a vhost task this means: > > > > - memory.high is handled at the wrong time. When a charge goes over > > memory.high, the task counts the pages, and should reclaim, and sleep > > if needed, on its way back to userspace. A vhost task only reclaims > > when more than a batch of pages has built up and the charge can block. > > Then it reclaims and sleeps in the middle of the charge, maybe while > > it holds a lock. vhost-scsi, for example, can fault in guest memory > > while it holds vq->mutex. If the charges can't block, nothing is done > > at all, and the count keeps growing. > > > > - Block cgroup throttling never runs. blkcg_schedule_throttle() asks for > > it, but only resume_user_mode_work() acts on it. > > > > - Task work queued with TWA_RESUME, like a deferred fput() or a NUMA > > balancing scan, waits until the task gets a signal or exits. > > > > io_uring workers had the same problem, and io_run_task_work() calls > > resume_user_mode_work() for them. Do the same in vhost_task_fn(): if > > TIF_NOTIFY_RESUME is set, call resume_user_mode_work() at the top of the > > loop. That is between work items, no locks are held there, and the task > > is running. Passing a NULL regs is fine, io_uring and KVM already do it. > > > > This covers every user of vhost_task_create(): the vhost workers and > > KVM's NX huge page recovery thread. > > > > Tested in a VM with a version of tools/virtio/vhost_net_test that puts > > each RX buffer on a new page, so the vhost worker faults in, and > > charges, 256M while its cgroup has memory.high=64M. Before, all 285 > > calls to __mem_cgroup_handle_over_high() came from try_charge_memcg(), > > inside handle_rx() with vq->mutex held. After, all 589 came from > > vhost_task_fn(). memory.peak stayed at 64M, and the run took about the > > same time (0.66s before, 0.67s after). A lockdep kernel showed no > > warnings. > > > > Signed-off-by: Shakeel Butt > > Is this different from: > > Message-ID: Oh wow, it is same but somewhat for different reasons. I am fine that patch, I will comment on it.