Messages in this thread | | | From | Greentime Hu <> | Date | Tue, 26 Oct 2021 12:44:31 +0800 | Subject | Re: [RFC PATCH v8 09/21] riscv: Add task switch support for vector |
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Darius Rad <darius@bluespec.com> 於 2021年10月26日 週二 上午12:22寫道: > > On Mon, Oct 25, 2021 at 12:47:49PM +0800, Greentime Hu wrote: > > Darius Rad <darius@bluespec.com> 於 2021年10月22日 週五 下午6:40寫道: > > > > > > On Fri, Oct 22, 2021 at 11:52:01AM +0800, Vincent Chen wrote: > > > > On Thu, Oct 21, 2021 at 6:50 PM Darius Rad <darius@bluespec.com> wrote: > > > > > > > > > > On Wed, Oct 20, 2021 at 06:01:31PM -0700, Paul Walmsley wrote: > > > > > > Hello Darius, > > > > > > > > > > > > On Tue, 5 Oct 2021, Darius Rad wrote: > > > > > > > > > > > > > On Mon, Oct 04, 2021 at 08:36:30PM +0800, Greentime Hu wrote: > > > > > > > > Darius Rad <darius@bluespec.com> 於 2021年9月29日 週三 下午9:28寫道: > > > > > > > > > > > > > > > > > > On Tue, Sep 28, 2021 at 10:56:52PM +0800, Greentime Hu wrote: > > > > > > > > > > Darius Rad <darius@bluespec.com> 於 2021年9月13日 週一 下午8:21寫道: > > > > > > > > > > > > > > > > > > > > > > On 9/8/21 1:45 PM, Greentime Hu wrote: > > > > > > > > > > > > This patch adds task switch support for vector. It supports partial lazy > > > > > > > > > > > > save and restore mechanism. It also supports all lengths of vlen. > > > > > > > > > > > > [ ... ] > > > > > > > > > > > > > > > > > So this will unconditionally enable vector instructions, and allocate > > > > > > > > > > > memory for vector state, for all processes, regardless of whether vector > > > > > > > > > > > instructions are used? > > > > > > > > > > > > > > > > > > > > Yes, it will enable vector if has_vector() is true. The reason that we > > > > > > > > > > choose to enable and allocate memory for user space program is because > > > > > > > > > > we also implement some common functions in the glibc such as memcpy > > > > > > > > > > vector version and it is called very often by every process. So that > > > > > > > > > > we assume if the user program is running in a CPU with vector ISA > > > > > > > > > > would like to use vector by default. If we disable it by default and > > > > > > > > > > make it trigger the illegal instruction, that might be a burden since > > > > > > > > > > almost every process will use vector glibc memcpy or something like > > > > > > > > > > that. > > > > > > > > > > > > > > > > > > Do you have any evidence to support the assertion that almost every process > > > > > > > > > would use vector operations? One could easily argue that the converse is > > > > > > > > > true: no existing software uses the vector extension now, so most likely a > > > > > > > > > process will not be using it. > > > > > > > > > > > > > > > > Glibc ustreaming is just starting so you didn't see software using the > > > > > > > > vector extension now and this patchset is testing based on those > > > > > > > > optimized glibc too. Vincent Chen is working on the glibc vector > > > > > > > > support upstreaming and we will also upstream the vector version glibc > > > > > > > > memcpy, memcmp, memchr, memmove, memset, strcmp, strlen. Then we will > > > > > > > > see platform with vector support can use vector version mem* and str* > > > > > > > > functions automatically based on ifunc and platform without vector > > > > > > > > will use the original one automatically. These could be done to select > > > > > > > > the correct optimized glibc functions by ifunc mechanism. > > > > > > > > > > > > In your reply, I noticed that you didn't address Greentime's response > > > > > > here. But this looks like the key issue. If common library functions are > > > > > > vector-accelerated, wouldn't it make sense that almost every process would > > > > > > wind up using vector instructions? And thus there wouldn't be much point > > > > > > to skipping the vector context memory allocation? > > > > > > > > > > > > > > > > This issue was addressed in the thread regarding Intel AMX I linked to in a > > > > > previous message. I don't agree that this is the key issue; it is one of a > > > > > number of issues. What if I don't want to take the potential > > > > > power/frequency hit for the vector unit for a workload that, at best, uses > > > > > it for the occasional memcpy? What if the allocation fails, how will that > > > > > > > > Hi Darius, > > > > The memcpy function seems not to be occasionally used in the programs > > > > because many functions in Glibc use memcpy() to complete the memory > > > > copy. I use the following simple case as an example. > > > > test.c > > > > void main(void) { > > > > return; > > > > } > > > > Then, we compile it by "gcc test.c -o a.out" and execute it. In the > > > > execution, the memcpy() has been called unexpectedly. It is because > > > > many libc initialized functions will be executed before entering the > > > > user-defined main function. One of the example is __libc_setup_tls(), > > > > which is called by __libc_start_main(). The __libc_setup_tls() will > > > > use memcpy() during the process of creating the Dynamic Thread Vector > > > > (DTV). > > > > > > > > Therefore, I think the memcpy() is widely used in most programs. > > > > > > > > > > You're missing my point. Not every (any?) program spends a majority of the > > > time doing memcpy(), and even if a program did, all of my points are still > > > valid. > > > > > > Please read the discussion in the thread I referenced and the questions in > > > my prior message. > > > > > > > Hi Darius, > > > > As I mentioned before, we want to treat vector ISA like a general ISA > > instead of a specific IP. User program should be able to use it > > transparently just like FPU. > > It seems that the use case you want is asking user to use vector like > > a specific IP, user program should ask kernel before they use it and > > that is not what we want to do in this patchset. > > > > Hi Greentime, > > Right. > > But beyond what I want to do or what you want to do, is what *should* Linux > do? I have attempted to provide evidence to support my position. You have > not responded to or addressed the majority of my questions, which is > concerning to me.
Hi Darius,
What is your majority questions?
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