Re: [RFC PATCH v8 09/21] riscv: Add task switch support for vector

[Darius Rad]

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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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
get reported to user space (hint: not well)?  According to Greentime,
RISC-V vector is similar to ARM SVE, which allocates memory for context
state on first use and not unconditionally for all processes.

// darius