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SubjectRe: [PATCH] sched, s390: Fix the fallout of increasing the offset of 'thread_struct' within 'task_struct'
On Mon, 20 Jul 2015 10:38:47 +0200
Ingo Molnar <mingo@kernel.org> wrote:

>
> * Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
>
> > diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
> > index 3238893..84062e7 100644
> > --- a/arch/s390/kernel/entry.S
> > +++ b/arch/s390/kernel/entry.S
> > @@ -178,17 +178,21 @@ _PIF_WORK = (_PIF_PER_TRAP)
> > */
> > ENTRY(__switch_to)
> > stmg %r6,%r15,__SF_GPRS(%r15) # store gprs of prev task
> > - stg %r15,__THREAD_ksp(%r2) # store kernel stack of prev
> > - lg %r4,__THREAD_info(%r2) # get thread_info of prev
> > - lg %r5,__THREAD_info(%r3) # get thread_info of next
> > + lgr %r1,%r2
> > + aghi %r1,__TASK_thread # thread_struct of prev task
> > + lg %r4,__TASK_thread_info(%r2) # get thread_info of prev
> > + lg %r5,__TASK_thread_info(%r3) # get thread_info of next
> > + stg %r15,__THREAD_ksp(%r1) # store kernel stack of prev
> > + lgr %r1,%r3
> > + aghi %r1,__TASK_thread # thread_struct of next task
> > lgr %r15,%r5
> > aghi %r15,STACK_INIT # end of kernel stack of next
> > stg %r3,__LC_CURRENT # store task struct of next
> > stg %r5,__LC_THREAD_INFO # store thread info of next
> > stg %r15,__LC_KERNEL_STACK # store end of kernel stack
> > + lg %r15,__THREAD_ksp(%r1) # load kernel stack of next
> > lctl %c4,%c4,__TASK_pid(%r3) # load pid to control reg. 4
> > mvc __LC_CURRENT_PID+4(4,%r0),__TASK_pid(%r3) # store pid of next
> > - lg %r15,__THREAD_ksp(%r3) # load kernel stack of next
> > lmg %r6,%r15,__SF_GPRS(%r15) # load gprs of next task
> > br %r14
>
> Btw., I think we'd be slightly better off with the variant I sent: that way the
> offset arithmetics are done in C code and are ready in the relevant registers by
> the time they are used in __switch_to().

If we save any instruction then not many, we are trading additional parameter
passing vs. four additional instructions in __switch_to. With this variant
the fallout is kept at a minimum which I would prefer at this point.

> >
> > @@ -417,6 +421,7 @@ ENTRY(pgm_check_handler)
> > LAST_BREAK %r14
> > lg %r15,__LC_KERNEL_STACK
> > lg %r14,__TI_task(%r12)
> > + aghi %r14,__TASK_thread # pointer to thread_struct
> > lghi %r13,__LC_PGM_TDB
> > tm __LC_PGM_ILC+2,0x02 # check for transaction abort
> > jz 2f
>
> Don't we also need the chunk I have:
>
> @@ -448,6 +449,7 @@ ENTRY(pgm_check_handler)
> nill %r10,0x007f
> sll %r10,2
> je .Lsysc_return
> + ahi %r14,-__TASK_thread_struct # r14 now points to 'current'
> lgf %r1,0(%r10,%r1) # load address of handler routine
> lgr %r2,%r11 # pass pointer to pt_regs
> basr %r14,%r1 # branch to interrupt-handler
>
> Because the 'basr' line relies on 'r14' having task_struct, I think?
>
> But I don't really know what I'm talking about here ...

The basr instruction is one of the function calling instructions and
%r14 is used for the return address. Between function calls the register
is basically free. So far it is used for the task_struct pointer
(the code following the "lg %r14,__TI_task(%r12)"). What I noticed is
that the pointer is only used to access the embedded thread_struct.
To get from the task_struct to a thread_struct a single aghi is
needed. The rest is moving stuff around in asm-offsets.c

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.



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