LOR on nfs: vfs_vnops.c:301 kern_descrip.c:1580

[pluknet]

On 19 August 2010 17:34, John Baldwin <jhb at freebsd.org> wrote:
> On Thursday, August 19, 2010 5:29:25 am pluknet wrote:
>> On 19 August 2010 00:07, John Baldwin <jhb at freebsd.org> wrote:
>> > On Wednesday, August 18, 2010 3:17:56 pm pluknet wrote:
>> >> On 18 August 2010 23:11, pluknet <pluknet at gmail.com> wrote:
>> >> > On 18 August 2010 17:46, Kostik Belousov <kostikbel at gmail.com> wrote:
>> >> >> On Wed, Aug 18, 2010 at 02:43:19PM +0400, pluknet wrote:
>> >> >>> On 18 August 2010 12:07, pluknet <pluknet at gmail.com> wrote:
>> >> >>> > On 17 August 2010 20:04, Kostik Belousov <kostikbel at gmail.com> wrote:
>> >> >>> >
>> >> >>> >>
>> >> >>> >> Also please take a note of the John' suggestion to use the taskqueue.
>> >> >>> >
>> >> >>> > I decided to go this road. Thank you both.
>> >> >>> > Now I do nfs buildkernel survive and prepare some benchmark results.
>> >> >>> >
>> >> >>>
>> >> >>> So, I modified the patch to defer proc_create() with taskqueue(9).
>> >> >>> Below is `time make -j5 buildkernel WITHOUT_MODULES=yes` perf.
>> > evaluation.
>> >> >>> Done on 4-way CPU on clean /usr/obj with /usr/src & /usr/obj both
>> >> >>> nfs-mounted over 1Gbit LAN.
>> >> >>>
>> >> >>> clean old
>> >> >>> 1137.985u 239.411s 7:42.15 298.0%       6538+2133k 87+43388io 226pf+0w
>> >> >>>
>> >> >>> clean new
>> >> >>> 1134.755u 240.032s 7:41.25 298.0%       6553+2133k 87+43367io 224pf+0w
>> >> >>>
>> >> >>> Patch needs polishing, though it generally works.
>> >> >>> Not sure if shep_chan (or whatever name it will get) needs locking.
>> >> >> As I said yesterday, if several requests to create nfsiod coming one
>> >> >> after another, you would loose all but the last.
>> >> >>
>> >> >> You should put the requests into the list, probably protected by
>> >> >> nfs_iod_mtx.
>> >> >>
>> >> >
>> >> > How about this patch? Still several things to ask.
>> >> > 1) I used malloc instance w/ M_NOWAIT, since it's called with nfs_iod_mtx
>> > held.
>> >> > 2) Probably busy/done gymnastics is a wrong mess. Your help is
>> > appreciated.
>> >> > 3) if (1) is fine, is it right to use fail: logic (i.e. set
>> >> > NFSIOD_NOT_AVAILABLE)
>> >> > on memory shortage? Not tested.
>> >> >
>> >> > There are debug printf() left intentionally to see how 3 contexts run
>> > under load
>> >> > to each other. I attached these messages as well if that makes sense.
>> >> >
>> >>
>> >> Ah, yes. Sorry, forgot about that.
>> >
>> > Your task handler needs to run in a loop until the list of requests is empty.
>> > If multiple threads call taskqueue_enqueue() before your task is scheduled,
>> > they will be consolidated down to a single call of your task handler.
>>
>> Thanks for your suggestions.
>>
>> So I converted nfs_nfsiodnew_tq() into loop, and as such
>> I changed a cleanup SLIST_FOREACH() as well to free a bulk of
>> (potentially consolidated) completed requests in one pass.
>> kproc_create() is still out of the SLIST_FOREACH() to avoid calling it
>> with nfs_iod_mtx held.
>>
>> >
>> > -       int error, i;
>> > -       int newiod;
>> > +       int i, newiod, error;
>> >
>> > You should sort these alphabetically if you are going to change this.  I would
>> > probably just leave it as-is.
>>
>> Err.. that's resulted after a set of changes. Thanks for noting that.
>>
>> >
>> > Also, I'm not sure how this works as you don't actually wait for the task to
>> > complete.  If the taskqueue_enqueue() doesn't preempt, then you may read
>> > ni_error as zero before the kproc has actually been created and return success
>> > even though an nfsiod hasn't been created.
>> >
>>
>> I put taskqueue_drain() right after taskqueue_enqueue() to be in sync with
>> task handler. That was part to think about, as I didn't find such a use pattern.
>> It seems though that tasks are launched now strictly one-by-one, without
>> visible parallelism (as far as debug printf()s don't overlap anymore).
>
> Ah, if it is safe to sleep then I have a couple of suggestions:
>

Cool, credits go to John :).
I just adopted all of your changes (attached).

> - Use M_WAITOK to malloc() so you don't have to worry about the failure case
>  (I see Rick already suggested this).

After all that reduces to the following replacement in nfs_nfsiodnew().
So, no regression should be there in theory.

 mtx_unlock(&nfs_iod_mtx);
- kproc_create(...)
+ malloc(...)
mtx_lock(&nfs_iod_mtx);

It survived after this simple test running for an hour, which
forces creation of many iods with r/w from 300 threads:

nfsserv:/home/svn/freebsd/obj_exp on /usr/obj (nfs)

./randomio /usr/obj/file 100 0.5 0 4096 60 100 &
./randomio /usr/obj/file 100 0.5 0 4096 60 100 &
./randomio /usr/obj/file 100 0.5 0 4096 60 100 &
while [ true ]; do sysctl vfs.nfs.iodmax=2; sysctl vfs.nfs.iodmax=60;
sleep 20; done

randomio is from ports/149838.


P.S.
it's funny to see in top
    0 root       10  44    0     0K   144K deadlk  2  23:16 28.86% kernel
Someone might think the kernel is in deadlock.

-- 
wbr,
pluknet
-------------- next part --------------
A non-text attachment was scrubbed...
Name: nfsiod_tq_lock.3.diff
Type: application/octet-stream
Size: 3680 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-current/attachments/20100820/a0190b40/nfsiod_tq_lock.3.obj