linux-kernel - Re: WARNING in _

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Message-ID: <11802a8a-ce41-f427-63d5-b6a4cf96bb3f@redhat.com>
Date:   Thu, 25 Jul 2019 22:25:25 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     syzbot <syzbot+e58112d71f77113ddb7b@...kaller.appspotmail.com>,
        aarcange@...hat.com, akpm@...ux-foundation.org,
        christian@...uner.io, davem@...emloft.net, ebiederm@...ssion.com,
        elena.reshetova@...el.com, guro@...com, hch@...radead.org,
        james.bottomley@...senpartnership.com, jglisse@...hat.com,
        keescook@...omium.org, ldv@...linux.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-parisc@...r.kernel.org,
        luto@...capital.net, mhocko@...e.com, mingo@...nel.org,
        namit@...are.com, peterz@...radead.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        wad@...omium.org
Subject: Re: WARNING in __mmdrop


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