Hi Stefane,
Thanks for the nice writeup. Some comments.
- Are there actually 2 syscalls each for the _pmrs functions?
- For attach_session, I think target needs to be an unsigned long, not
an int.
- I need delete_eventset functionality. Dynamic tools set up and tear
down eventsets, multiplexing, sampling, etc... How can I do it with the
new code?
Thanks,
Phil
stephane eranian wrote:
> Hello,
>
> As you may know, I have been working on redesigning the perfmon2 system call
> API to address some issues raised by the LKML review a couple of months back.
>
> I have made some good progress on this and I'd like to share the current
> design
> with you.
>
> There were three major goals in the redesign:
>
> - minimize the number of system calls. Pefmon2 v2.x has 12 syscalls. This
> was
> considered quite a lot for a single subsystem.
>
> - make sure we could extend the syscall API without necessarily
> adding new syscalls.
> That means having flexibility in each syscall and also in the
> structures passed
> in syscalls.
>
> - make sure it would be possible to use existing v2.0 (IA-64) and
> also v2.x (x>0)
> applications with, at worse, a recompilation. Unlike v2.0 (IA-64
> single syscall API)
> the compatibility would have to be provided by user level code,
> e.g., libpfm.
> Obviously statically linked applications would have to be recompiled.
>
> I am happy to report that I has successfully fulfilled those three
> goals. Here is how
> the new API looks like now:
>
> - down to 8 system calls, all with different names from v2.81
> - several data structures, pfarg_*, have been removed
> - full compatibility with v2.81 except for one system call
> (pfm_delete_evtsets)
> - all system calls are extensible via a flags parameter
> - all structures have reserved fields
> - the context abstraction has been replaced with the notion of a session.
>
> Here are the details:
>
> I) session creation
>
> With v2.81:
> int pfm_create_context(pfarg_ctx_t *ctx, char *smpl_name, void
> *smpl_arg, size_t smpl_size);
>
> With v3.0:
> int pfm_create_session(int flags);
> int pfm_create_session(int flags, char *smpl_name, void
> *smpl_arg, size_t smpl_size);
>
> New Flags:
> PFM_FL_SMPL_FMT : indicate using sampling format and
> that 3 additional parameters are passed
>
> The pfarg_ctx_t structure has been abandoned. The flags parameter is
> used very much like for the open(2)
> syscall to indicate that additional (optional) parameters are passed.
>
> All old flags are preserved.
>
> The call still returns the file descriptor uniquely identifying the
> session.
>
> Just like with context, a session can either be monitoring a thread or a
> CPU.
>
> II) programming the registers
>
> With v2.81:
> int pfm_write_pmcs(int fd, pfarg_pmc_t *pmds, int n);
> int pfm_write_pmds(int fd, pfarg_pmd_t *pmcs, int n);
> int pfm_read_pmds(int fd, parg_pmd_t *pmds, int n);
>
> With v3.0:
> int pfm_write_pmrs(int fd, int flags, pfarg_pmr_t *pmrs, int n);
> int pfm_write_pmrs(int fd, int flags, pfarg_pmr_t *pmrs, int n,
> pfarg_pmd_attr_t *pmas);
>
> int pfm_read_pmrs(int fd, int flags, pfarg_pmr_t *pmrs, int n);
> int pfm_read_pmrs(int fd, int flags, parg_pmr_t *pmrs, int n,
> pfarg_pmd_attr_t *pmas);
>
> New structures:
>
> typedef struct {
> u16 reg_num;
> u16 reg_set;
> u32 reg_flags;
> u64 reg_value;
> } pfarg_pmr_t;
>
> typedef struct {
> u64 reg_long_reset;
> u64 reg_short_reset;
> u64 reg_random_mask;
> u64 reg_smpl_pmds[PFM_PMD_BV];
> u64 reg_reset_pmds[PFM_PMD_BV];
> u64 reg_ovfl_swcnt;
> u64 reg_smpl_eventid;
> u64 reg_last_value;
> u64 reg_reserved[8];
> };
>
> New flags:
> PFM_RWFL_PMD : pmrs contains PMD register descriptions
> PFM_RWFL_PMC : pmrs contains PMC register descriptions
> PFM_RWFL_PMD_ATTR: an additional vector is passed in pmas
>
> We now use only 2 system calls to read and write the PMU registers.
> This is possible because
> we are sharing the same register description data structure,
> pfarg_pmr_t. They key attributes
> of each register are encapsulated into this structure. Additional
> PMD attributes related to sampling
> and multiplexing are off-loaded into another optional structure,
> pfarg_pmd_attr_t. This structure
> becomes optional and is only looked at by the kernel if the
> PFM_RWFL_PMD_ATTR flag is passed.
>
> For all counting applications, using pfarg_pmr_t is enough. The nice
> side effect of this split is that
> the cost of reading and writing PMD register is now reduced because
> we have less data to copy in
> and out of the kernel.
>
> Unlike suggested by some people, I have not merged the notions of
> PMD and PMC registers. I think
> it is cleaner to separate them out. It also makes it much easier to
> provide backward compatibility with
> v2.81.
>
> III) attaching and detaching
>
> With v2.81:
> int pfm_load_context(int fd, pfarg_load_t *load);
> int pfm_unload_context(int fd);
>
> With v3.0:
> int pfm_attach_session(int fd, int flags, int target);
> int pfm_detach_session(int fd, int flags);
>
> The pfarg_load_t structure has been abandoned. The information about what
> to
> attach to is passed as a parameter to the syscall in "target". It
> can either be
> a thread id or a CPU id.
>
> There are currently no flags defined for either call.
>
> Note that we have lost the ability to specify which event set is
> to be activated first.
> There was no actual use of this option anyway.
>
> IV) starting and stopping
>
> With v2.81:
> int pfm_start(int fd, pfarg_start_t *st);
> int pfm_stop(int fd);
> int pfm_restart(int fd);
>
> With v3.0:
> int pfm_start_session(int fd, int flags);
> int pfm_stop_session(int fd, int flags);
>
> New flags:
> PFM_STFL_RESTART: resume monitoring after an overflow notification
>
> The pfarg_start_t structure has been abandoned.
>
> The pfm_restart() syscall has been merged with pfm_start() by
> using the PFM_STFL_RESTART
> flag. It is not possible to just use pfm_start_session() and
> internally determine what to do because
> this is dependent on the sampling format.
>
> We have lost the ability to specify on which event set to
> start. I don't think this option was ever
> used.
>
> V) event set and multiplexing
>
> With v2.81:
> int pfm_create_evtsets(int fd, pfarg_setdesc_t *s, int n);
> int pfm_getinfo_evtsets(int fd, pfarg_setinfo_t *s, int n);
> int pfm_delete_evtsets(int fd, pfarg_setdesc_t *s, int n);
>
> With v3.0:
> int pfm_create_sets(int fd, int flags, pfarg_setdesc_t *s, int n);
> int pfm_getinfo_evtsets(int fd, int flags, pfarg_setinfo_t *s, int
> n);
>
> We have kept the same data structures and simply added a flags
> parameters to provide
> for extensibility of the calls.
>
> We have removed pfm_delete_evtsets() because it was not used by a
> lot of applications.
> We could add it back later if there is a good reason for it,
> something stronger than saying
> it needs to be there for symmetry.
>
> VI) libpfm
>
> The libpfm library will support v2.81 and v3.0 transparently. The
> examples have all been rewritten
> to the new API. The v2.81 examples are still there for testing
> and comparison purposes. They
> compile with no modifications.
>
> The perfmon.h header has been strongly restructured to accomodate
> v3.0, v2.81 and older v2.x
> releases as used by Cray and SiCortex or even older v2.0 IA-64 programs.
>
> When using a the v2.81 API, the library is able to adapt based on
> the host kernel perfmon version.
> If the host kernel is using v2.81 then the calls are simply
> passed through. If instead, the kernel
> uses v3.0, then each v2.81 call is mapped onto its equivalent
> v3.0 whenever possible. If the
> syscall has no equivalent, e.g., pfm_delete_evtsets(), then an
> error is returned. The errno
> value is updated correctly when using v2.81 <-> v3.0 glue layer.
>
> VII) conclusions
>
> There are no other modifications to other parts of the API.
>
> I think this ultimate modification addresses ALL outstanding
> issues raised by LKML.
>
> I have presented the new API for a fully featured perfmon, with
> sampling, system-wide and multiplexing.
> This is not what is going to go into the mainline kernel
> initially. The plan is to use this as the development
> code and pull bits and pieces into the minimal kernel patch that
> I maintain separately and which is
> what I intend to post to LKML. The reason I modified the fully
> featured version is to gauge all the changes
> needed and their potential connections.
>
> I will soon, create branches on both the GIT tree, libpfm CVS so
> that you will be able to experiment with this
> API. The v3.0 has been added to all architectures currently
> supported. They all compile. I was not able to
> tests many of them for lack of hardware.
>
> I welcome any comments you may have about this new API.
>
> S.Eranian
>
> Thanks.
>
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