Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: WARNING in __mmdrop
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@syzkaller.appspotmail.com>,
        aarcange@redhat.com, akpm@linux-foundation.org,
        christian@brauner.io, davem@davemloft.net, ebiederm@xmission.com,
        elena.reshetova@intel.com, guro@fb.com, hch@infradead.org,
        james.bottomley@hansenpartnership.com, jglisse@redhat.com,
        keescook@chromium.org, ldv@altlinux.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-parisc@vger.kernel.org,
        luto@amacapital.net, mhocko@suse.com, mingo@kernel.org,
        namit@vmware.com, peterz@infradead.org,
        syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk,
        wad@chromium.org
References: <20190723032024-mutt-send-email-mst@kernel.org>
 <1d14de4d-0133-1614-9f64-3ded381de04e@redhat.com>
 <20190723035725-mutt-send-email-mst@kernel.org>
 <3f4178f1-0d71-e032-0f1f-802428ceca59@redhat.com>
 <20190723051828-mutt-send-email-mst@kernel.org>
 <caff362a-e208-3468-3688-63e1d093a9d3@redhat.com>
 <20190725012149-mutt-send-email-mst@kernel.org>
 <55e8930c-2695-365f-a07b-3ad169654d28@redhat.com>
 <20190725042651-mutt-send-email-mst@kernel.org>
 <84bb2e31-0606-adff-cf2a-e1878225a847@redhat.com>
 <20190725092332-mutt-send-email-mst@kernel.org>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <11802a8a-ce41-f427-63d5-b6a4cf96bb3f@redhat.com>
Date:   Thu, 25 Jul 2019 22:25:25 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.8.0
MIME-Version: 1.0
In-Reply-To: <20190725092332-mutt-send-email-mst@kernel.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 2019/7/25 下午9:26, Michael S. Tsirkin wrote:
>> Exactly, and that's the reason actually I use synchronize_rcu() there.
>>
>> So the concern is still the possible synchronize_expedited()?
> I think synchronize_srcu_expedited.
>
> synchronize_expedited sends lots of IPI and is bad for realtime VMs.
>
>> Can I do this
>> on through another series on top of the incoming V2?
>>
>> Thanks
>>
> The question is this: is this still a gain if we switch to the
> more expensive srcu? If yes then we can keep the feature on,


I think we only care about the cost on srcu_read_lock() which looks 
pretty tiny form my point of view. Which is basically a READ_ONCE() + 
WRITE_ONCE().

Of course I can benchmark to see the difference.


> if not we'll put it off until next release and think
> of better solutions. rcu->srcu is just a find and replace,
> don't see why we need to defer that. can be a separate patch
> for sure, but we need to know how well it works.


I think I get here, let me try to do that in V2 and let's see the numbers.

Thanks