[Ping x3] Re: [PATCH, nvptx, 1/2] Reimplement libgomp barriers for nvptx

[Chung-Lin Tang via Gcc-patches]

Ping x3.

On 2022/10/31 10:18 PM, Chung-Lin Tang wrote:
> Ping x2.
> 
> On 2022/10/17 10:29 PM, Chung-Lin Tang wrote:
>> Ping.
>>
>> On 2022/9/21 3:45 PM, Chung-Lin Tang via Gcc-patches wrote:
>>> Hi Tom,
>>> I had a patch submitted earlier, where I reported that the current way of 
>>> implementing
>>> barriers in libgomp on nvptx created a quite significant performance drop 
>>> on some SPEChpc2021
>>> benchmarks:
>>> https://gcc.gnu.org/pipermail/gcc-patches/2022-September/600818.html
>>>
>>> That previous patch wasn't accepted well (admittedly, it was kind of a 
>>> hack).
>>> So in this patch, I tried to (mostly) re-implement team-barriers for NVPTX.
>>>
>>> Basically, instead of trying to have the GPU do CPU-with-OS-like things 
>>> that it isn't suited for,
>>> barriers are implemented simplistically with bar.* synchronization 
>>> instructions.
>>> Tasks are processed after threads have joined, and only if team->task_count 
>>> != 0
>>>
>>> (arguably, there might be a little bit of performance forfeited where 
>>> earlier arriving threads
>>> could've been used to process tasks ahead of other threads. But that again 
>>> falls into requiring
>>> implementing complex futex-wait/wake like behavior. Really, that kind of 
>>> tasking is not what target
>>> offloading is usually used for)
>>>
>>> Implementation highlight notes:
>>> 1. gomp_team_barrier_wake() is now an empty function (threads never "wake" 
>>> in the usual manner)
>>> 2. gomp_team_barrier_cancel() now uses the "exit" PTX instruction.
>>> 3. gomp_barrier_wait_last() now is implemented using "bar.arrive"
>>>
>>> 4. gomp_team_barrier_wait_end()/gomp_team_barrier_wait_cancel_end():
>>>     The main synchronization is done using a 'bar.red' instruction. This 
>>> reduces across all threads
>>>     the condition (team->task_count != 0), to enable the task processing 
>>> down below if any thread
>>>     created a task. (this bar.red usage required the need of the second GCC 
>>> patch in this series)
>>>
>>> This patch has been tested on x86_64/powerpc64le with nvptx offloading, 
>>> using libgomp, ovo, omptests,
>>> and sollve_vv testsuites, all without regressions. Also verified that the 
>>> SPEChpc 2021 521.miniswp_t
>>> and 534.hpgmgfv_t performance regressions that occurred in the GCC12 cycle 
>>> has been restored to
>>> devel/omp/gcc-11 (OG11) branch levels. Is this okay for trunk?
>>>
>>> (also suggest backporting to GCC12 branch, if performance regression can be 
>>> considered a defect)
>>>
>>> Thanks,
>>> Chung-Lin
>>>
>>> libgomp/ChangeLog:
>>>
>>> 2022-09-21  Chung-Lin Tang  <clt...@codesourcery.com>
>>>
>>>     * config/nvptx/bar.c (generation_to_barrier): Remove.
>>>     (futex_wait,futex_wake,do_spin,do_wait): Remove.
>>>     (GOMP_WAIT_H): Remove.
>>>     (#include "../linux/bar.c"): Remove.
>>>     (gomp_barrier_wait_end): New function.
>>>     (gomp_barrier_wait): Likewise.
>>>     (gomp_barrier_wait_last): Likewise.
>>>     (gomp_team_barrier_wait_end): Likewise.
>>>     (gomp_team_barrier_wait): Likewise.
>>>     (gomp_team_barrier_wait_final): Likewise.
>>>     (gomp_team_barrier_wait_cancel_end): Likewise.
>>>     (gomp_team_barrier_wait_cancel): Likewise.
>>>     (gomp_team_barrier_cancel): Likewise.
>>>     * config/nvptx/bar.h (gomp_team_barrier_wake): Remove
>>>     prototype, add new static inline function.