Re: [PATCH 4.4 48/76] libceph: force GFP_NOIO for socket allocations

[Ilya Dryomov]

On Wed, Mar 29, 2017 at 12:55 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Wed 29-03-17 12:41:26, Michal Hocko wrote:
> [...]
>> > ceph_con_workfn
>> >   mutex_lock(&con->mutex)  # ceph_connection::mutex
>> >   try_write
>> >     ceph_tcp_connect
>> >       sock_create_kern
>> >         GFP_KERNEL allocation
>> >           allocator recurses into XFS, more I/O is issued
>
> One more note. So what happens if this is a GFP_NOIO request which
> cannot make any progress? Your IO thread is blocked on con->mutex
> as you write below but the above thread cannot proceed as well. So I am
> _really_ not sure this acutally helps.

This is not the only I/O worker.  A ceph cluster typically consists of
at least a few OSDs and can be as large as thousands of OSDs.  This is
the reason we are calling sock_create_kern() on the writeback path in
the first place: pre-opening thousands of sockets isn't feasible.

Thanks,

                Ilya