Re: [UNIKRAFT PATCH v2 1/2] build: Add PGO support

[Razvan Deaconescu <razvan.deaconescu@xxxxxxxxx>]

On Mon, Nov 23, 2020 at 1:23 AM Alice Suiu <alicesuiu17@xxxxxxxxx> wrote:
> În sâm., 21 nov. 2020 la 18:15, Razvan Deaconescu 
> <razvan.deaconescu@xxxxxxxxx> a scris:
>> Hi, Alice, Simon.
>>
>> I applied these patches and it went OK to generate PGO files (the
>> .gcda file generated by gcov).
>>
>> I've two remarks:
>> * LDFLAGS is used for linking, not for compiling. I don't see why
>> $(COMPFLAGS) should be here. Maybe it should actually be CFLAGS. The
>> -fprofile-generate option needs to be part of both the compiling and
>> linking phases, while the -fprofile-use option needs to only be part
>> of the compiling phase.
>> * The application / library Makefile needs to be appended with something like
>> ```
>> APPHELLOWORLD_CFLAGS-$(CONFIG_OPTIMIZE_PGO_GENERATE) += -fprofile-generate
>> APPHELLOWORLD_CFLAGS-$(CONFIG_OPTIMIZE_PGO_USE) += -fprofile-use
>> ```
>> ** Firstly, this ^ needs to be documented in the source code, maybe in
>> the README file.
>> ** Secondly, why is this required? Why won't simply selecting the
>> CONFIG_OPTIMIZE_PGO_... option append the compiling / linking flags
>> for all software components (including the Unikernel) and that's that?
>> Is this a limitation of the build system, or is it an approach to
>> granularly configure profile generation per software component?
>
> Hi Razvan,
>
> The reason I had to use the profile-generate flag only when compiling the 
> application to be executed in Unikraft was that the functions in the libgcov 
> library use structures as TLS and TCB. Thus, if I used the profile-generate 
> flag to compile all the libraries, including the Unikernel, I would receive 
> errors.

This is unclear to me. What's the connection between the use of TLS
and TCB and the use of the -fprofile-generate flag. And the receiving
of errors.

> So, before calling each function in the system, a profiling function is 
> called from the libgcov library, and this function needed the allocated TLS 
> and TCB structures.

Razvan

>> On Mon, Jun 8, 2020 at 12:08 PM Alice Suiu <alicesuiu17@xxxxxxxxx> wrote:
>> >
>> > Profile-Guided Optimization is a method used by compilers to produce
>> > optimal code by using application runtime data. PGO involves two
>> > phases one that implies a compilation and a run that will generate
>> > the profiling files for the application and one that implies a
>> > compilation in which the compiler uses the profiling data to make a
>> > better estimate of the program’s control flow and to produce an optimized
>> > executable file.
>> >
>> > Add the configuration for PGO phases in MENUCONFIG and the correspondent
>> > flag for LDFLAGS.
>> >
>> > Signed-off-by: Alice Suiu <alicesuiu17@xxxxxxxxx>
>> > ---
>> >  Config.uk   | 26 ++++++++++++++++++++++++++
>> >  Makefile.uk |  8 ++++++++
>> >  2 files changed, 34 insertions(+)
>> >
>> > diff --git a/Config.uk b/Config.uk
>> > index 3235b91..a815b6b 100644
>> > --- a/Config.uk
>> > +++ b/Config.uk
>> > @@ -86,6 +86,32 @@ config OPTIMIZE_LTO
>> >                 will increase overall building time but creates more 
>> > efficient
>> >                 Unikraft binaries.
>> >
>> > +config OPTIMIZE_PGO
>> > +       bool "PGO - Profile-guided optimization"
>> > +       default n
>> > +       help
>> > +               Enables GCC to do code optimizations through PGO.
>> > +
>> > +if OPTIMIZE_PGO
>> > +       choice PGO_PHASES
>> > +       prompt "PGO Options"
>> > +       default OPTIMIZE_PGO_GENERATE
>> > +       help
>> > +               Enables GCC to do code optimizations through PGO.
>> > +
>> > +       config OPTIMIZE_PGO_GENERATE
>> > +               bool "profile-generate"
>> > +               help
>> > +                       Enables GCC to generate data for code optimization 
>> > through PGO.
>> > +
>> > +       config OPTIMIZE_PGO_USE
>> > +               bool "profile-use"
>> > +               help
>> > +                       Enables GCC to do code optimizations using data 
>> > generated from
>> > +                       the profile-generated phase of PGO.
>> > +       endchoice
>> > +endif
>> > +
>> >  choice
>> >         prompt "Debug information level"
>> >         default DEBUG_SYMBOLS_LVL3
>> > diff --git a/Makefile.uk b/Makefile.uk
>> > index a78afc0..78d802c 100644
>> > --- a/Makefile.uk
>> > +++ b/Makefile.uk
>> > @@ -36,6 +36,14 @@ COMPFLAGS-$(CONFIG_OPTIMIZE_LTO)          += -flto
>> >  LIBLDFLAGS-$(CONFIG_OPTIMIZE_LTO)         += $(COMPFLAGS) $(COMPFLAGS-y) 
>> > $(ARCHFLAGS) $(ARCHFLAGS-y)
>> >  LDFLAGS-$(CONFIG_OPTIMIZE_LTO)            += $(COMPFLAGS) $(COMPFLAGS-y) 
>> > $(ARCHFLAGS) $(ARCHFLAGS-y)
>> >
>> > +# PGO - profile-generate phase - requires the -fprofile-generate flag
>> > +# when compiling the application and linking all libraries in the system.
>> > +LDFLAGS-$(CONFIG_OPTIMIZE_PGO_GENERATE)   += $(COMPFLAGS) $(COMPFLAGS-y) 
>> > $(ARCHFLAGS) $(ARCHFLAGS-y) -fprofile-generate
>> > +
>> > +# PGO - profile-use phase - requires the -fprofile-use flag
>> > +# when compiling the application and linking all libraries in the system.
>> > +LDFLAGS-$(CONFIG_OPTIMIZE_PGO_USE)        += $(COMPFLAGS) $(COMPFLAGS-y) 
>> > $(ARCHFLAGS) $(ARCHFLAGS-y) -fprofile-use
>> > +
>> >  DBGFLAGS-$(CONFIG_DEBUG_SYMBOLS_LVL0)     += -g0
>> >  DBGFLAGS-$(CONFIG_DEBUG_SYMBOLS_LVL1)     += -g1
>> >  DBGFLAGS-$(CONFIG_DEBUG_SYMBOLS_LVL2)     += -g2
>> > --
>> > 2.17.1
>> >
>> >