Date:	Fri, 2 Dec 2011 03:47:27 +0000
From:	Shi Xuelin-B29237 <B29237@...escale.com>
To:	"Ira W. Snyder" <iws@...o.caltech.edu>
CC:	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Li Yang-R58472 <r58472@...escale.com>
Subject: RE: [PATCH][RFC] fsldma: fix performance degradation by optimizing
 spinlock use.

Hi Iris,

>I'm convinced that "smp_rmb()" is needed when removing the spinlock. As noted, Documentation/memory-barriers.txt says that stores on one CPU can be
>observed by another CPU in a different order.
>Previously, there was an UNLOCK (in fsl_dma_tx_submit) followed by a LOCK (in fsl_tx_status). This provided a "full barrier", forcing the operations to 
>complete correctly when viewed by the second CPU. 

I do not agree this smp_rmb() works here. Because when this smp_rmb() executed and begin to read chan->common.cookie, you still cannot avoid the order issue. Something like one is reading old value, but another CPU is updating the new value. 

My point is here the order is not important for the DMA decision.
Completed DMA tx is decided as not complete is not a big deal, because next time it will be OK.

I believe there is no case that could cause uncompleted DMA tx is decided as completed, because the fsl_tx_status is called after fsl_dma_tx_submit for a specific cookie. If you can give me an example here, I will agree with you.

Thanks,
Forrest 

-----Original Message-----
From: Ira W. Snyder [mailto:iws@...o.caltech.edu] 
Sent: 2011年12月1日 1:08
To: Shi Xuelin-B29237
Cc: vinod.koul@...el.com; dan.j.williams@...el.com; linuxppc-dev@...ts.ozlabs.org; linux-kernel@...r.kernel.org; Li Yang-R58472
Subject: Re: [PATCH][RFC] fsldma: fix performance degradation by optimizing spinlock use.

On Wed, Nov 30, 2011 at 09:57:47AM +0000, Shi Xuelin-B29237 wrote:
> Hello Ira,
> 
> In drivers/dma/dmaengine.c, we have below tight loop to check DMA completion in mainline Linux:
>        do {
>                 status = dma_async_is_tx_complete(chan, cookie, NULL, NULL);
>                 if (time_after_eq(jiffies, dma_sync_wait_timeout)) {
>                         printk(KERN_ERR "dma_sync_wait_timeout!\n");
>                         return DMA_ERROR;
>                 }
>         } while (status == DMA_IN_PROGRESS);
> 

That is the body of dma_sync_wait(). It is mostly used in the raid code.
I understand that you don't want to change the raid code to use callbacks.

In any case, I think we've strayed from the topic under consideration, which is: can we remove this spinlock without introducing a bug.

I'm convinced that "smp_rmb()" is needed when removing the spinlock. As noted, Documentation/memory-barriers.txt says that stores on one CPU can be observed by another CPU in a different order.

Previously, there was an UNLOCK (in fsl_dma_tx_submit) followed by a LOCK (in fsl_tx_status). This provided a "full barrier", forcing the operations to complete correctly when viewed by the second CPU. From the
text:

> Therefore, from (1), (2) and (4) an UNLOCK followed by an 
> unconditional LOCK is equivalent to a full barrier, but a LOCK followed by an UNLOCK is not.

Also, please read "EXAMPLES OF MEMORY BARRIER SEQUENCES" and "INTER-CPU LOCKING BARRIER EFFECTS". Particularly, in "EXAMPLES OF MEMORY BARRIER SEQUENCES", the text notes:

> Without intervention, CPU 2 may perceive the events on CPU 1 in some 
> effectively random order, despite the write barrier issued by CPU 1:
>
> [snip diagram]
>
> And thirdly, a read barrier acts as a partial order on loads. Consider 
> the following sequence of events:
>
> [snip diagram]
>
> Without intervention, CPU 2 may then choose to perceive the events on 
> CPU 1 in some effectively random order, despite the write barrier issued by CPU 1:
>
> [snip diagram]
>

And so on. Please read this entire section in the document.

I can't give you an ACK on the proposed patch. To the best of my understanding, I believe it introduces a bug. I've tried to provide as much evidence for this belief as I can, in the form of documentation in the kernel source tree. If you can cite some documentation that shows I am wrong, I will happily change my mind!

Ira

> -----Original Message-----
> From: Ira W. Snyder [mailto:iws@...o.caltech.edu]
> Sent: 2011年11月30日 1:26
> To: Li Yang-R58472
> Cc: Shi Xuelin-B29237; vinod.koul@...el.com; dan.j.williams@...el.com; 
> linuxppc-dev@...ts.ozlabs.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH][RFC] fsldma: fix performance degradation by optimizing spinlock use.
> 
> On Tue, Nov 29, 2011 at 03:19:05AM +0000, Li Yang-R58472 wrote:
> > > Subject: Re: [PATCH][RFC] fsldma: fix performance degradation by 
> > > optimizing spinlock use.
> > > 
> > > On Thu, Nov 24, 2011 at 08:12:25AM +0000, Shi Xuelin-B29237 wrote:
> > > > Hi Ira,
> > > >
> > > > Thanks for your review.
> > > >
> > > > After second thought, I think your scenario may not occur.
> > > > Because the cookie 20 we query must be returned by
> > > > fsl_dma_tx_submit(...) in
> > > practice.
> > > > We never query a cookie not returned by fsl_dma_tx_submit(...).
> > > >
> > > 
> > > I agree about this part.
> > > 
> > > > When we call fsl_tx_status(20), the chan->common.cookie is 
> > > > definitely wrote as
> > > 20 and cpu2 could not read as 19.
> > > >
> > > 
> > > This is what I don't agree about. However, I'm not an expert on CPU cache vs.
> > > memory accesses in an multi-processor system. The section titled 
> > > "CACHE COHERENCY" in Documentation/memory-barriers.txt leads me to 
> > > believe that the scenario I described is possible.
> > 
> > For Freescale PowerPC, the chip automatically takes care of cache coherency.  Even if this is a concern, spinlock can't address it.
> > 
> > > 
> > > What happens if CPU1's write of chan->common.cookie only goes into 
> > > CPU1's cache. It never makes it to main memory before CPU2 fetches the old value of 19.
> > > 
> > > I don't think you should see any performance impact from the
> > > smp_mb() operation.
> > 
> > Smp_mb() do have impact on performance if it's in the hot path.  While it might be safer having it, I doubt it is really necessary.  If the CPU1 doesn't have the updated last_used, it's shouldn't have known there is a cookie 20 existed either.
> > 
> 
> I believe that you are correct, for powerpc. However, anything outside of arch/powerpc shouldn't assume it only runs on powerpc. I wouldn't be surprised to see fsldma running on an iMX someday (ARM processor).
> 
> My interpretation says that the change introduces the possibility that
> fsl_tx_status() returns the wrong answer for an extremely small time window, on SMP only, based on Documentation/memory-barriers.txt. But I can't seem convince you.
> 
> My real question is what code path is hitting this spinlock? Is it in mainline Linux? Why is it polling rather than using callbacks to determine DMA completion?
> 
> Thanks,
> Ira
> 
> > > > -----Original Message-----
> > > > From: Ira W. Snyder [mailto:iws@...o.caltech.edu]
> > > > Sent: 2011年11月23日 2:59
> > > > To: Shi Xuelin-B29237
> > > > Cc: dan.j.williams@...el.com; Li Yang-R58472; zw@...kernel.org; 
> > > > vinod.koul@...el.com; linuxppc-dev@...ts.ozlabs.org; 
> > > > linux-kernel@...r.kernel.org
> > > > Subject: Re: [PATCH][RFC] fsldma: fix performance degradation by 
> > > > optimizing
> > > spinlock use.
> > > >
> > > > On Tue, Nov 22, 2011 at 12:55:05PM +0800, b29237@...escale.com wrote:
> > > > > From: Forrest Shi <b29237@...escale.com>
> > > > >
> > > > >     dma status check function fsl_tx_status is heavily called in
> > > > >     a tight loop and the desc lock in fsl_tx_status contended by
> > > > >     the dma status update function. this caused the dma performance
> > > > >     degrades much.
> > > > >
> > > > >     this patch releases the lock in the fsl_tx_status function.
> > > > >     I believe it has no neglect impact on the following call of
> > > > >     dma_async_is_complete(...).
> > > > >
> > > > >     we can see below three conditions will be identified as success
> > > > >     a)  x < complete < use
> > > > >     b)  x < complete+N < use+N
> > > > >     c)  x < complete < use+N
> > > > >     here complete is the completed_cookie, use is the last_used
> > > > >     cookie, x is the querying cookie, N is MAX cookie
> > > > >
> > > > >     when chan->completed_cookie is being read, the last_used may
> > > > >     be incresed. Anyway it has no neglect impact on the dma status
> > > > >     decision.
> > > > >
> > > > >     Signed-off-by: Forrest Shi <xuelin.shi@...escale.com>
> > > > > ---
> > > > >  drivers/dma/fsldma.c |    5 -----
> > > > >  1 files changed, 0 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/drivers/dma/fsldma.c b/drivers/dma/fsldma.c index 
> > > > > 8a78154..1dca56f 100644
> > > > > --- a/drivers/dma/fsldma.c
> > > > > +++ b/drivers/dma/fsldma.c
> > > > > @@ -986,15 +986,10 @@ static enum dma_status 
> > > > > fsl_tx_status(struct
> > > dma_chan *dchan,
> > > > >  	struct fsldma_chan *chan = to_fsl_chan(dchan);
> > > > >  	dma_cookie_t last_complete;
> > > > >  	dma_cookie_t last_used;
> > > > > -	unsigned long flags;
> > > > > -
> > > > > -	spin_lock_irqsave(&chan->desc_lock, flags);
> > > > >
> > > >
> > > > This will cause a bug. See below for a detailed explanation. You need this instead:
> > > >
> > > > 	/*
> > > > 	 * On an SMP system, we must ensure that this CPU has seen the
> > > > 	 * memory accesses performed by another CPU under the
> > > > 	 * chan->desc_lock spinlock.
> > > > 	 */
> > > > 	smp_mb();
> > > > >  	last_complete = chan->completed_cookie;
> > > > >  	last_used = dchan->cookie;
> > > > >
> > > > > -	spin_unlock_irqrestore(&chan->desc_lock, flags);
> > > > > -
> > > > >  	dma_set_tx_state(txstate, last_complete, last_used, 0);
> > > > >  	return dma_async_is_complete(cookie, last_complete, 
> > > > > last_used);  }
> > > >
> > > > Facts:
> > > > - dchan->cookie is the same member as chan->common.cookie (same 
> > > > memory
> > > > location)
> > > > - chan->common.cookie is the "last allocated cookie for a pending transaction"
> > > > - chan->completed_cookie is the "last completed transaction"
> > > >
> > > > I have replaced "dchan->cookie" with "chan->common.cookie" in 
> > > > the below
> > > explanation, to keep everything referenced from the same structure.
> > > >
> > > > Variable usage before your change. Everything is used locked.
> > > > - RW chan->common.cookie		(fsl_dma_tx_submit)
> > > > - R  chan->common.cookie		(fsl_tx_status)
> > > > - R  chan->completed_cookie		(fsl_tx_status)
> > > > - W  chan->completed_cookie		(dma_do_tasklet)
> > > >
> > > > Variable usage after your change:
> > > > - RW chan->common.cookie		LOCKED
> > > > - R  chan->common.cookie		NO LOCK
> > > > - R  chan->completed_cookie		NO LOCK
> > > > - W  chan->completed_cookie             LOCKED
> > > >
> > > > What if we assume that you have a 2 CPU system (such as a P2020). 
> > > > After your
> > > changes, one possible sequence is:
> > > >
> > > > === CPU1 - allocate + submit descriptor: fsl_dma_tx_submit() === 
> > > > spin_lock_irqsave
> > > > descriptor->cookie = 20		(x in your example)
> > > > chan->common.cookie = 20	(used in your example)
> > > > spin_unlock_irqrestore
> > > >
> > > > === CPU2 - immediately calls fsl_tx_status() ===
> > > > chan->common.cookie == 19
> > > > chan->completed_cookie == 19
> > > > descriptor->cookie == 20
> > > >
> > > > Since we don't have locks anymore, CPU2 may not have seen the 
> > > > write to
> > > > chan->common.cookie yet.
> > > >
> > > > Also assume that the DMA hardware has not started processing the 
> > > > transaction yet. Therefore dma_do_tasklet() has not been called, 
> > > > and
> > > > chan->completed_cookie has not been updated.
> > > >
> > > > In this case, dma_async_is_complete() (on CPU2) returns 
> > > > DMA_SUCCESS, even
> > > though the DMA operation has not succeeded. The DMA operation has 
> > > not even started yet!
> > > >
> > > > The smp_mb() fixes this, since it forces CPU2 to have seen all 
> > > > memory operations
> > > that happened before CPU1 released the spinlock. Spinlocks are 
> > > implicit SMP memory barriers.
> > > >
> > > > Therefore, the above example becomes:
> > > > smp_mb();
> > > > chan->common.cookie == 20
> > > > chan->completed_cookie == 19
> > > > descriptor->cookie == 20
> > > >
> > > > Then dma_async_is_complete() returns DMA_IN_PROGRESS, which is correct.
> > > >
> > > > Thanks,
> > > > Ira
> > > >
> > > >
> > > > _______________________________________________
> > > > Linuxppc-dev mailing list
> > > > Linuxppc-dev@...ts.ozlabs.org
> > > > https://lists.ozlabs.org/listinfo/linuxppc-dev
> > 
> 

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