Ideas/suggestions to avoid repeated locking and reducing too many lists with dmaengine?

Just wanted your thoughts/suggestions on how we can avoid overhead in the EDMA dmaengine driver. I am seeing a lots of performance drop specially for small transfers with EDMA versus before raw EDMA was moved to DMAEngine framework (atleast 25%).

One of the things I am thinking about is the repeated (spin) locking/unlocking of the virt_dma_chan->lock or vc->lock. In many cases, there's only 1 user or thread requiring to do a DMA, so I feel the locking is unnecessary and potential overhead. If there's a sane way to detect this an avoid locking altogether, that would be great.

Also with respect to virt_dma (which is used by edma to manage all the
descriptors and lists) there are too many lists: submitted, issued, completed etc and the descriptor moves from one to the other. I am thinking if there is a way we can avoid using so many lists and just have 2 lists and move the desc from one list to the other, That could avoid using the intermediate list altogether and classify dma requests as "done" or "not done".

Wrapping... (I've had to manually edit this.)

Quoted 11 lines · Joel Fernandes

Just wanted your thoughts/suggestions on how we can avoid overhead in the EDMA dmaengine driver. I am seeing a lots of performance drop specially for small transfers with EDMA versus before raw EDMA was moved to DMAEngine framework (atleast 25%).

One of the things I am thinking about is the repeated (spin) locking/unlocking of the virt_dma_chan->lock or vc->lock. In many cases, there's only 1 user or thread requiring to do a DMA, so I feel the locking is unnecessary and potential overhead. If there's a sane way to detect this an avoid locking altogether, that would be great.

For the case where there's no contention, spinlocks /should/ be light. What will make them more expensive is if you have things like lockdep enabled, which adds much more code into those paths to do state tracking. It's a known side effect of using that debug.

So, if you're developing, then you should always have turned lockdep on. If you're testing for performance, you should have lockdep and spinlock debugging turned off.

Quoted 8 lines

Also with respect to virt_dma (which is used by edma to manage all the
descriptors and lists) there are too many lists: submitted, issued, completed etc and the descriptor moves from one to the other. I am thinking if there is a way we can avoid using so many lists and just have 2 lists and move the desc from one list to the other, That could avoid using the intermediate list altogether and classify dma requests as "done" or "not done".

The reason I created separate submitted and issued lists is that it's much easier to manage than having everything on a single list.

We could deal with the submitted vs issued list, and that's to have the channel store the cookie for the last issued descriptor - but I wonder if it's worth the effort.

What I'd suggest is to try some profiling, and post some profiling results which show where the problems are, rather than pointing at bits of code you might not particularly like.

Quoted 11 lines

Quoted 7 lines

Also with respect to virt_dma (which is used by edma to manage all the
descriptors and lists) there are too many lists: submitted, issued, completed etc and the descriptor moves from one to the other. I am thinking if there is a way we can avoid using so many lists and just have 2 lists and move the desc from one list to the other, That could avoid using the intermediate list altogether and classify dma requests as "done" or "not done".

The reason I created separate submitted and issued lists is that it's much easier to manage than having everything on a single list.

Quoted 8 lines

We could deal with the submitted vs issued list, and that's to have the channel store the cookie for the last issued descriptor - but I wonder if it's worth the effort.

What I'd suggest is to try some profiling, and post some profiling results which show where the problems are, rather than pointing at bits of code you might not particularly like.

Actually I did do some tracing earlier before I posted this thread- and notice there was excessive traces of locking/unlocking. It is very light though as you pointed and lighter without debug options. The only other notable difference is the fact that we are now going through the dmaengine framework in the newer kernel vs the faster one.

One more thing in my trace is omap_dma_sync repeatedly call in memcpy_to_io for every barrier call which is not necessary. I am working on a fix this.

On turning off DEBUG_KERNEL and running more tests, I do see some improvements however the throughput reduction is still =~ 10%

With a modified openssl speed test app, I sent 16-byte sized block repeatedly to the AES crypto hardware accelerator using EDMA:

On v3.13.5 kernel:
root at am335x-evm:~# openssl speed -evp aes-128-cbc -engine cryptodev engine "cryptodev" set.
Doing aes-128-cbc for 3s on 16 size blocks: 79902 aes-128-cbc's

With v3.2 kernel,
Doing aes-128-cbc for 3s on 16 size blocks: 92314 aes-128-cbc's

So we're able to encrypt around 13k more ops, or around 4.5k ops/second with 3.13.5

Actually I did do some tracing earlier before I posted this thread- and notice there was excessive traces of locking/unlocking. It is very light though as you pointed and lighter without debug options. The only other notable difference is the fact that we are now going through the dmaengine framework in the newer kernel vs the faster one.

Hi folks,

Just wanted your thoughts/suggestions on how we can avoid overhead in the EDMA dmaengine driver. I am seeing a lots of performance drop specially for small transfers with EDMA versus before raw EDMA was moved to DMAEngine framework (atleast 25%).

One of the things I am thinking about is the repeated (spin) locking/unlocking of the virt_dma_chan->lock or vc->lock. In many cases, there's only 1 user or thread requiring to do a DMA, so I feel the locking is unnecessary and potential overhead. If there's a sane way to detect this an avoid locking altogether, that would be great.

Quoted 6 lines · Joel Fernandes

Hi folks,

Just wanted your thoughts/suggestions on how we can avoid overhead in the EDMA dmaengine driver. I am seeing a lots of performance drop specially for small transfers with EDMA versus before raw EDMA was moved to DMAEngine framework (atleast 25%).

I've seen roughly the same drop in my testing. In my case it had to do with the nature of how work is done using virt-dma. The virt-dma is predicated on only letting one transaction be active at a time and it increases the latency for getting the next transaction off. For large transactions, it's negligible. But for small transactions, it is pretty evident.