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

[Chung-Lin Tang <cltang@codesourcery.com>]

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  <cltang@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.