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

[Ley Foon Tan]

On Mon, Oct 4, 2021 at 8:41 PM Greentime Hu <greentime.hu at sifive.com> wrote:
>
> Ley Foon Tan <lftan.linux at gmail.com> 於 2021年10月1日 週五 上午10:46寫道：
> >
> > On Wed, Sep 29, 2021 at 11:54 PM Darius Rad <darius at bluespec.com> wrote:
> > >
> > > On Tue, Sep 28, 2021 at 10:56:52PM +0800, Greentime Hu wrote:
> > > > Darius Rad <darius at bluespec.com> 於 2021年9月13日 週一 下午8:21寫道：
> > > > >
[....]


> > > > >
> > > > > So this will unconditionally enable vector instructions, and allocate
> > > > > memory for vector state, for all processes, regardless of whether vector
> > > > > instructions are used?
> > > > >
> > > >
> > > > Hi Darius,
> > > >
> > > > 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.
> > >
> > > >
> > > > > Given the size of the vector state and potential power and performance
> > > > > implications of enabling the vector engine, it seems like this should
> > > > > treated similarly to Intel AMX on x86.  The full discussion of that is
> > > > > here:
> > > > >
> > > > > https://lore.kernel.org/lkml/CALCETrW2QHa2TLvnUuVxAAheqcbSZ-5_WRXtDSAGcbG8N+gtdQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org/
> > > > >
> > > > > The cover letter for recent Intel AMX patches has a summary of the x86
> > > > > implementation:
> > > > >
> > > > > https://lore.kernel.org/lkml/20210825155413.19673-1-chang.seok.bae@intel.com/
> > > > >
> > > > > If RISC-V were to adopt a similar approach, I think the significant
> > > > > points are:
> > > > >
> > > > >   1. A process (or thread) must specifically request the desire to use
> > > > > vector extensions (perhaps with some new arch_prctl() API),
> > > > >
> > > > >   2. The kernel is free to deny permission, perhaps based on
> > > > > administrative rules or for other reasons, and
> > > > >
> > > > >   3. If a process attempts to use vector extensions before doing the
> > > > > above, the process will die due to an illegal instruction.
> > > >
> > > > Thank you for sharing this, but I am not sure if we should treat
> > > > vector like AMX on x86. IMHO, compiler might generate code with vector
> > > > instructions automatically someday, maybe we should treat vector
> > > > extensions like other extensions.
> > > > If user knows the vector extension is supported in this CPU and he
> > > > would like to use it, it seems we should let user use it directly just
> > > > like other extensions.
> > > > If user don't know it exists or not, user should use the library API
> > > > transparently and let glibc or other library deal with it. The glibc
> > > > ifunc feature or multi-lib should be able to choose the correct
> > > > implementation.
> > >
> > > What makes me think that the vector extension should be treated like AMX is
> > > that they both (1) have a significant amount of architectural state, and
> > > (2) likely have a significant power and/or area impact on (non-emulated)
> > > designs.
> > >
> > > For example, I think it is possible, maybe even likely, that vector
> > > implementations will have one or more of the following behaviors:
> > >
> > >   1. A single vector unit shared among two or more harts,
> > >
> > >   2. Additional power consumption when the vector unit is enabled and idle
> > > versus not being enabled at all,
> > >
> > >   3. For a system which supports variable operating frequency, a reduction
> > > in the maximum frequency when the vector unit is enabled, and/or
> > >
> > >   4. The inability to enter low power states and/or delays to low power
> > > states transitions when the vector unit is enabled.
> > >
> > > None of the above constraints apply to more ordinary extensions like
> > > compressed or the various bit manipulation extensions.
> > >
> > > The discussion I linked to has some well reasoned arguments on why
> > > substantial extensions should have a mechanism to request using them by
> > > user space.  The discussion was in the context of Intel AMX, but applies to
> > > further x86 extensions, and I think should also apply to similar extensions
> > > on RISC-V, like vector here.
> > >
> > There is possible use case where not all cores support vector
> > extension due to size, area and power.
> > Perhaps can have the mechanism or flow to determine the
> > application/thread require vector extension or it specifically request
> > the desire to use
> > vector extensions. Then this app/thread run on cpu with vector
> > extension capability only.
> >
>
> IIRC, we assume all harts has the same ability in Linux because of SMP
> assumption.
> If we have more information of hw capability and we may use this
> information for scheduler to switch the correct process to the correct
> CPU.
> Do you have any idea how to implement it in Linux kernel? Maybe we can
> list in the TODO list.
I think we can refer to other arch implementations as reference:

1. ARM64 supports 32-bit thread on asymmetric AArch32 systems. There
is a flag in ELF to check, then start the thread on the core that
supports 32-bit execution. This patchset is merged to mainline 5.15.
https://lore.kernel.org/linux-arm-kernel/20210730112443.23245-8-will@kernel.org/T/

2. Link shared by Darius, on-demand request implementation on Intel AMX
https://lore.kernel.org/lkml/20210825155413.19673-1-chang.seok.bae@intel.com/

glibc support optimized library functions with vector, this is enabled
by default if compiler is with vector extension enabled? If yes, then
most of the app required vector core.

Regards
Ley Foon