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Date: Wed, 18 Jul 2012 09:22:48 +0800
From: Asias He <asias@...hat.com>
To: Jeff Moyer <jmoyer@...hat.com>
CC: linux-kernel@...r.kernel.org,
"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RESEND 5/5] vhost-blk: Add vhost-blk support
On 07/18/2012 03:10 AM, Jeff Moyer wrote:
> Asias He <asias@...hat.com> writes:
>
>> vhost-blk is a in kernel virito-blk device accelerator.
>>
>> This patch is based on Liu Yuan's implementation with various
>> improvements and bug fixes. Notably, this patch makes guest notify and
>> host completion processing in parallel which gives about 60% performance
>> improvement compared to Liu Yuan's implementation.
>
> So, first off, some basic questions. Is it correct to assume that you
> tested this with buffered I/O (files opened *without* O_DIRECT)?
> I'm pretty sure that if you used O_DIRECT, you'd run into problems (which
> are solved by the patch set posted by Shaggy, based on Zach Brown's work
> of many moons ago). Note that, with buffered I/O, the submission path
> is NOT asynchronous. So, any speedups you've reported are extremely
> suspect. ;-)
I always used O_DIRECT to test this patchset. And I mostly used raw
block device as guest image. Is this the reason why I did not hit the
problem you mentioned. Btw, I do have run this patchset on image based
file. I still do not see problems like IO hangs.
> Next, did you look at Shaggy's patch set? I think it would be best to
> focus your efforts on testing *that*, and implementing your work on top
> of it.
I guess you mean this one:
http://marc.info/?l=linux-fsdevel&m=133312234313122
I did not notice that until James pointed that out.
I talked with Zach and Shaggy. Shaggy said he is still working on that
patch set and will send that patch out soon.
> Having said that, I did do some review of this patch, inlined below.
Thanks, Jeff!
>> +static int vhost_blk_setup(struct vhost_blk *blk)
>> +{
>> + struct kioctx *ctx;
>> +
>> + if (blk->ioctx)
>> + return 0;
>> +
>> + blk->ioevent_nr = blk->vq.num;
>> + ctx = ioctx_alloc(blk->ioevent_nr);
>> + if (IS_ERR(ctx)) {
>> + pr_err("Failed to ioctx_alloc");
>> + return PTR_ERR(ctx);
>> + }
>> + put_ioctx(ctx);
>> + blk->ioctx = ctx;
>> +
>> + blk->ioevent = kmalloc(sizeof(struct io_event) * blk->ioevent_nr,
>> + GFP_KERNEL);
>> + if (!blk->ioevent) {
>> + pr_err("Failed to allocate memory for io_events");
>> + return -ENOMEM;
>
> You've just leaked blk->ioctx.
Yes. Will fix.
>> + }
>> +
>> + blk->reqs = kmalloc(sizeof(struct vhost_blk_req) * blk->ioevent_nr,
>> + GFP_KERNEL);
>> + if (!blk->reqs) {
>> + pr_err("Failed to allocate memory for vhost_blk_req");
>> + return -ENOMEM;
>
> And here.
Yes. Will fix.
>
>> + }
>> +
>> + return 0;
>> +}
>> +
> [snip]
>> +static int vhost_blk_io_submit(struct vhost_blk *blk, struct file *file,
>> + struct vhost_blk_req *req,
>> + struct iovec *iov, u64 nr_vecs, loff_t offset,
>> + int opcode)
>> +{
>> + struct kioctx *ioctx = blk->ioctx;
>> + mm_segment_t oldfs = get_fs();
>> + struct kiocb_batch batch;
>> + struct blk_plug plug;
>> + struct kiocb *iocb;
>> + int ret;
>> +
>> + if (!try_get_ioctx(ioctx)) {
>> + pr_info("Failed to get ioctx");
>> + return -EAGAIN;
>> + }
>
> Using try_get_ioctx directly gives me a slightly uneasy feeling. I
> understand that you don't need to do the lookup, but at least wrap it
> and check for ->dead.
OK.
>
>> +
>> + atomic_long_inc_not_zero(&file->f_count);
>> + eventfd_ctx_get(blk->ectx);
>> +
>> + /* TODO: batch to 1 is not good! */
>
> Agreed. You should setup the batching in vhost_blk_handle_guest_kick.
> The way you've written the code, the batching is not at all helpful.
Yes. that's why there is a TODO.
>> + kiocb_batch_init(&batch, 1);
>> + blk_start_plug(&plug);
>> +
>> + iocb = aio_get_req(ioctx, &batch);
>> + if (unlikely(!iocb)) {
>> + ret = -EAGAIN;
>> + goto out;
>> + }
>> +
>> + iocb->ki_filp = file;
>> + iocb->ki_pos = offset;
>> + iocb->ki_buf = (void *)iov;
>> + iocb->ki_left = nr_vecs;
>> + iocb->ki_nbytes = nr_vecs;
>> + iocb->ki_opcode = opcode;
>> + iocb->ki_obj.user = req;
>> + iocb->ki_eventfd = blk->ectx;
>> +
>> + set_fs(KERNEL_DS);
>> + ret = aio_setup_iocb(iocb, false);
>> + set_fs(oldfs);
>> + if (unlikely(ret))
>> + goto out_put_iocb;
>> +
>> + spin_lock_irq(&ioctx->ctx_lock);
>> + if (unlikely(ioctx->dead)) {
>> + spin_unlock_irq(&ioctx->ctx_lock);
>> + ret = -EINVAL;
>> + goto out_put_iocb;
>> + }
>> + aio_run_iocb(iocb);
>> + spin_unlock_irq(&ioctx->ctx_lock);
>> +
>> + aio_put_req(iocb);
>> +
>> + blk_finish_plug(&plug);
>> + kiocb_batch_free(ioctx, &batch);
>> + put_ioctx(ioctx);
>> +
>> + return ret;
>> +out_put_iocb:
>> + aio_put_req(iocb); /* Drop extra ref to req */
>> + aio_put_req(iocb); /* Drop I/O ref to req */
>> +out:
>> + put_ioctx(ioctx);
>> + return ret;
>> +}
>> +
>
> You've duplicated a lot of io_submit_one. I'd rather see that factored
> out than to have to maintain two copies.
Agree.
> Again, what I'd *really* like to see is you rebase on top of Shaggy's
> work.
Sure. Let's wait for Shaggy's new version.
--
Asias
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