Re: [PATCH] ring: relax alignment constraint on ring structure

On 25-May-18 1:18 PM, Burakov, Anatoly wrote:

On 25-May-18 11:59 AM, Burakov, Anatoly wrote:

quoted

On 03-Apr-18 2:26 PM, Olivier Matz wrote:

quoted

The initial objective of
commit d9f0d3a1ffd4 ("ring: remove split cacheline build setting")
was to add an empty cache line betwee, the producer and consumer
data (on platform with cache line size = 64B), preventing from
having them on adjacent cache lines.

Following discussion on the mailing list, it appears that this
also imposes an alignment constraint that is not required.

This patch removes the extra alignment constraint and adds the
empty cache lines using padding fields in the structure. The
size of rte_ring structure and the offset of the fields remain
the same on platforms with cache line size = 64B:

   rte_ring = 384
   rte_ring.name = 0
   rte_ring.flags = 32
   rte_ring.memzone = 40
   rte_ring.size = 48
   rte_ring.mask = 52
   rte_ring.prod = 128
   rte_ring.cons = 256

But it has an impact on platform where cache line size is 128B:

   rte_ring = 384        -> 768
   rte_ring.name = 0
   rte_ring.flags = 32
   rte_ring.memzone = 40
   rte_ring.size = 48
   rte_ring.mask = 52
   rte_ring.prod = 128   -> 256
   rte_ring.cons = 256   -> 512

Link: http://dpdk.org/dev/patchwork/patch/25039/
Suggested-by: Konstantin Ananyev <redacted>
Signed-off-by: Olivier Matz <redacted>
---

This patch causes eal_flags_autotest to hang on FreeBSD. Root cause at 
this time is unknown, but this is a weird one - the test seems to hang 
while doing read() in bsd/eal_thread.c:59. Reverting this patch on top 
of rc5 results in eal_flags_autotest passing.

Further investigation shows that for some reason, if Enter is pressed 
while the test is seemingly "hung", it continues and passes. Or rather, 
it hangs on one test, and if Enter is pressed, it finishes that test and 
hangs on another, after which pressing Enter again will result in test 
succeeding. Weird...

This patch is OK, the problem is in the test :) With this patch, it 
apparently blew up in just the right way, but really, the EAL flags 
autotest is wrong on FreeBSD because it launches primary processes with 
'--no-shconf' option, which corrupt main process's memory. For some 
reason this only began manifesting with this patch.

-- 
Thanks,
Anatoly