linux-kernel - Re: [PATCH v2 8/8] powerpc: Enable Generic Entry/Exit for syscalls.

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Message-ID: <e6xsjevzubhemxrajz7kz4t2w3fwtkmybr6zd2a4v3qgf6y7kl@xqf4tddmu7cs>
Date: Tue, 16 Dec 2025 20:39:50 +0530
From: Mukesh Kumar Chaurasiya <mkchauras@...ux.ibm.com>
To: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
Cc: maddy@...ux.ibm.com, mpe@...erman.id.au, npiggin@...il.com,
        oleg@...hat.com, kees@...nel.org, luto@...capital.net,
        wad@...omium.org, mchauras@...ux.ibm.com, thuth@...hat.com,
        sshegde@...ux.ibm.com, charlie@...osinc.com, macro@...am.me.uk,
        akpm@...ux-foundation.org, ldv@...ace.io, deller@....de,
        ankur.a.arora@...cle.com, segher@...nel.crashing.org,
        tglx@...utronix.de, thomas.weissschuh@...utronix.de,
        peterz@...radead.org, menglong8.dong@...il.com, bigeasy@...utronix.de,
        namcao@...utronix.de, kan.liang@...ux.intel.com, mingo@...nel.org,
        atrajeev@...ux.vnet.ibm.com, mark.barnett@....com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 8/8] powerpc: Enable Generic Entry/Exit for syscalls.

On Tue, Dec 16, 2025 at 07:41:11AM +0100, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 14/12/2025 à 14:02, Mukesh Kumar Chaurasiya a écrit :
> > From: Mukesh Kumar Chaurasiya <mchauras@...ux.ibm.com>
> > 
> > Convert the PowerPC syscall entry and exit paths to use the generic
> > entry/exit framework by selecting GENERIC_ENTRY and integrating with
> > the common syscall handling routines.
> > 
> > This change transitions PowerPC away from its custom syscall entry and
> > exit code to use the generic helpers such as:
> >   - syscall_enter_from_user_mode()
> >   - syscall_exit_to_user_mode()
> > 
> > As part of this migration:
> >   - The architecture now selects GENERIC_ENTRY in Kconfig.
> >   - Old tracing, seccomp, and audit handling in ptrace.c is removed in
> >     favor of generic entry infrastructure.
> >   - interrupt.c and syscall.c are simplified to delegate context
> >     management and user exit handling to the generic entry path.
> >   - The new pt_regs field `exit_flags` introduced earlier is now used
> >     to carry per-syscall exit state flags (e.g. _TIF_RESTOREALL).
> > 
> > This aligns PowerPC with the common entry code used by other
> > architectures and reduces duplicated logic around syscall tracing,
> > context tracking, and signal handling.
> > 
> > The performance benchmarks from perf bench basic syscall are below:
> > 
> > perf bench syscall usec/op
> > 
> > | Test            | With Patch | Without Patch | % Change |
> > | --------------- | ---------- | ------------- | -------- |
> > | getppid usec/op | 0.207795   | 0.210373      | -1.22%   |
> > | getpgid usec/op | 0.206282   | 0.211676      | -2.55%   |
> > | fork usec/op    | 833.986    | 814.809       | +2.35%   |
> > | execve usec/op  | 360.939    | 365.168       | -1.16%   |
> > 
> > perf bench syscall ops/sec
> > 
> > | Test            | With Patch | Without Patch | % Change |
> > | --------------- | ---------- | ------------- | -------- |
> > | getppid ops/sec | 48,12,433  | 47,53,459     | +1.24%   |
> > | getpgid ops/sec | 48,47,744  | 47,24,192     | +2.61%   |
> > | fork ops/sec    | 1,199      | 1,227         | -2.28%   |
> > | execve ops/sec  | 2,770      | 2,738         | +1.16%   |
> 
> I get about 2% degradation on powerpc 8xx, and it is quite stable over time
> when repeating the test.
> 
> 'perf bench syscall all' on powerpc 8xx (usec per op):
> 
> | Test            | With Patch | Without Patch | % Change |
> | --------------- | ---------- | ------------- | -------- |
> | getppid usec/op | 2.63       | 2.63          | ~ 0%     |
> | getpgid usec/op | 2.26       | 2.22          | +2,80%   |
> | fork usec/op    | 15300      | 15000         | +2,00%   |
> | execve usec/op  | 45700      | 45200         | +1.10%   |
> 
Ohk,
Do you have any idea where we might be loosing performance? I don't have
an 8xx chip so i am not sure where to look.

Thanks for testing it out.

Regards,
Mukesh
> Christophe