Re: Possible RAID6 regression with ASYNC_TX_DMA enabled in 4.1

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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Hi Markus,

On Thu, May 07, 2015 at 02:39:07PM +0000, Markus Stockhausen wrote:
> Hi Maxime,
> 
> > Von: linux-raid-owner@vger.kernel.org [linux-raid-owner@vger.kernel.org]" im Auftrag von "Maxime Ripard [maxime.ripard@free-electrons.com]
> > Gesendet: Donnerstag, 7. Mai 2015 14:57
> > An: Neil Brown; Shaohua Li
> > Cc: linux-raid@vger.kernel.org; linux-kernel@vger.kernel.org; Lior Amsalem; Thomas Petazzoni; Gregory Clement; Boris Brezillon
> > Betreff: Possible RAID6 regression with ASYNC_TX_DMA enabled in 4.1
> > 
> > Hi,
> > 
> > I'm currently trying to add support for the PQ operations on the
> > marvell XOR engine, in dmaengine, obviously to be able to use async_tx
> > to offload these operations.
> > 
> > I'm testing these patches with a RAID6 array with 4 disks.
> > 
> > However, since the commit 59fc630b8b5f ("RAID5: batch adjacent full
> > stripe write", every write to that array fails with the following
> > stacktrace.
> > 
> > http://code.bulix.org/eh8iew-88342?raw
> 
> I don't know if it might be related. I added support for RAID6 Read-Modify-Write
> in software XOR with some patches. The following commit mangles some lines in 
> async_pq.c:
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?
> id=584acdd49cd2472ca0f5a06adbe979db82d0b4af
> 
> I introduced a new flag ASYNC_TX_PQ_XOR_DST that notifies the async layer
> that we want to do a XOR syndrome operation instead of a full calculation.
> This will enforce the software path because I guessed that hardware does not
> support that case. Without hardware to check I might have missed some 
> checks in the async layer.
> 
> In the upper layer ops_run_reconstruct6 will set the flag if we determined
> that rmw is faster than rcw.
> 
> Can you check if rmw_level=0 fixes the issue. See:
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?
> id=d06f191f8ecaef4d524e765fdb455f96392fbd42

I just gave this a try, and it doesn't fix anything.

One thing I forgot to mention is that our hardware doesn't support the
PQ multiplications and product sums, so one of the patches we have is
to add a new ASYNC_TX flag to be able to identify and bail out of such
transfers.

The patch is here:
https://github.com/MISL-EBU-System-SW/mainline-public/commit/9964fe4a79da10162f83bd527b3fe44da60d7e0f

There might be some interaction between your patch and this one, even
though the async_tx code itself looks to be untouched by your patches.

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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