Re: [RFC 2/2] x86_64: expand kernel stack to 16K

From: Konstantin Khlebnikov
Date: Tue Jun 03 2014 - 09:02:38 EST

On Sat, May 31, 2014 at 6:06 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 2014-05-28 20:42, Linus Torvalds wrote:
>>> Regardless of whether it is swap or something external queues the
>>> bio on the plug, perhaps we should look at why it's done inline
>>> rather than by kblockd, where it was moved because it was blowing
>>> the stack from schedule():
>> So it sounds like we need to do this for io_schedule() too.
>> In fact, we've generally found it to be a mistake every time we
>> "automatically" unblock some IO queue. And I'm not saying that because
>> of stack space, but because we've _often_ had the situation that eager
>> unblocking results in IO that could have been done as bigger requests.
> We definitely need to auto-unplug on the schedule path, otherwise we run
> into all sorts of trouble. But making it async off the IO schedule path is
> fine. By definition, it's not latency sensitive if we are hitting unplug on
> schedule. I'm pretty sure it was run inline on CPU concerns here, as running
> inline is certainly cheaper than punting to kblockd.
>> Looking at that callchain, I have to say that ext4 doesn't look
>> horrible compared to the whole block layer and virtio.. Yes,
>> "ext4_writepages()" is using almost 400 bytes of stack, and most of
>> that seems to be due to:
>> struct mpage_da_data mpd;
>> struct blk_plug plug;
> Plus blk_plug is pretty tiny as it is. I queued up a patch to kill the magic
> part of it, since that's never caught any bugs. Only saves 8 bytes, but may
> as well take that. Especially if we end up with nested plugs.

In case of nested plugs only the first one is used? Right?
So, it may be embedded into task_struct together with integer recursion counter.
This will save bit of precious stack and make it looks cleaner.

>> Well, we've definitely have had some issues with deeper callchains
>> with md, but I suspect virtio might be worse, and the new blk-mq code
>> is lilkely worse in this respect too.
> I don't think blk-mq is worse than the older stack, in fact it should be
> better. The call chains are shorter, and a lot less cruft on the stack.
> Historically the stack issues have been nested devices, however. And for
> sync IO, we do run it inline, so if the driver chews up a lot of stack,
> well...
> Looks like I'm late here and the decision has been made to go 16K stacks,
> which I think is a good one. We've been living on the edge (and sometimes
> over) for heavy dm/md setups for a while, and have been patching around that
> fact in the IO stack for years.
> --
> Jens Axboe
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