[+ Sukadev, Christophe]
On 18/04/18 11:08, Alastair D'Silva wrote:
From: Alastair D'Silva <alast...@d-silva.org>
The current implementation of TID allocation, using a global IDR, may
result in an errant process starving the system of available TIDs.
Instead, use task_pid_nr(), as mentioned by the original author. The
scenario described which prevented it's use is not applicable, as
set_thread_tidr can only be called after the task struct has been
populated.
Signed-off-by: Alastair D'Silva <alast...@d-silva.org>
So it's too late in the evening for me to completely get my head around
what's going on here enough to give my Reviewed-by:, but my current
thinking is:
- In the first version of the patch to add TIDR support
(https://patchwork.ozlabs.org/patch/799494/), it was originally proposed
to call assign_thread_id() (as it was then called) from copy_thread()
- The comment block documents the reason why we can't use task_pid_nr()
but assumes that we're trying to assign a TIDR from within copy_thread()
- The final patch that was accepted
(https://patchwork.ozlabs.org/patch/835552/,
ec233ede4c8654894610ea54f4dae7adc954ac62) instead sets the TIDR to 0
from copy_thread(), so the original reasoning regarding not using
task_pid_nr() within copy_thread() is no longer applicable.
Sukadev: does this sound right?
Andrew
---
arch/powerpc/include/asm/switch_to.h | 1 -
arch/powerpc/kernel/process.c | 97 +-----------------------------------
2 files changed, 1 insertion(+), 97 deletions(-)
diff --git a/arch/powerpc/include/asm/switch_to.h
b/arch/powerpc/include/asm/switch_to.h
index be8c9fa23983..5b03d8a82409 100644
--- a/arch/powerpc/include/asm/switch_to.h
+++ b/arch/powerpc/include/asm/switch_to.h
@@ -94,6 +94,5 @@ static inline void clear_task_ebb(struct task_struct *t)
extern int set_thread_uses_vas(void);
extern int set_thread_tidr(struct task_struct *t);
-extern void clear_thread_tidr(struct task_struct *t);
#endif /* _ASM_POWERPC_SWITCH_TO_H */
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 3b00da47699b..87f047fd2762 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1496,103 +1496,12 @@ int set_thread_uses_vas(void)
}
#ifdef CONFIG_PPC64
-static DEFINE_SPINLOCK(vas_thread_id_lock);
-static DEFINE_IDA(vas_thread_ida);
-
-/*
- * We need to assign a unique thread id to each thread in a process.
- *
- * This thread id, referred to as TIDR, and separate from the Linux's tgid,
- * is intended to be used to direct an ASB_Notify from the hardware to the
- * thread, when a suitable event occurs in the system.
- *
- * One such event is a "paste" instruction in the context of Fast Thread
- * Wakeup (aka Core-to-core wake up in the Virtual Accelerator Switchboard
- * (VAS) in POWER9.
- *
- * To get a unique TIDR per process we could simply reuse task_pid_nr() but
- * the problem is that task_pid_nr() is not yet available copy_thread() is
- * called. Fixing that would require changing more intrusive arch-neutral
- * code in code path in copy_process()?.
- *
- * Further, to assign unique TIDRs within each process, we need an atomic
- * field (or an IDR) in task_struct, which again intrudes into the arch-
- * neutral code. So try to assign globally unique TIDRs for now.
- *
- * NOTE: TIDR 0 indicates that the thread does not need a TIDR value.
- * For now, only threads that expect to be notified by the VAS
- * hardware need a TIDR value and we assign values > 0 for those.
- */
-#define MAX_THREAD_CONTEXT ((1 << 16) - 1)
-static int assign_thread_tidr(void)
-{
- int index;
- int err;
- unsigned long flags;
-
-again:
- if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL))
- return -ENOMEM;
-
- spin_lock_irqsave(&vas_thread_id_lock, flags);
- err = ida_get_new_above(&vas_thread_ida, 1, &index);
- spin_unlock_irqrestore(&vas_thread_id_lock, flags);
-
- if (err == -EAGAIN)
- goto again;
- else if (err)
- return err;
-
- if (index > MAX_THREAD_CONTEXT) {
- spin_lock_irqsave(&vas_thread_id_lock, flags);
- ida_remove(&vas_thread_ida, index);
- spin_unlock_irqrestore(&vas_thread_id_lock, flags);
- return -ENOMEM;
- }
-
- return index;
-}
-
-static void free_thread_tidr(int id)
-{
- unsigned long flags;
-
- spin_lock_irqsave(&vas_thread_id_lock, flags);
- ida_remove(&vas_thread_ida, id);
- spin_unlock_irqrestore(&vas_thread_id_lock, flags);
-}
-
-/*
- * Clear any TIDR value assigned to this thread.
- */
-void clear_thread_tidr(struct task_struct *t)
-{
- if (!t->thread.tidr)
- return;
-
- if (!cpu_has_feature(CPU_FTR_P9_TIDR)) {
- WARN_ON_ONCE(1);
- return;
- }
-
- mtspr(SPRN_TIDR, 0);
- free_thread_tidr(t->thread.tidr);
- t->thread.tidr = 0;
-}
-
-void arch_release_task_struct(struct task_struct *t)
-{
- clear_thread_tidr(t);
-}
-
/*
* Assign a unique TIDR (thread id) for task @t and set it in the thread
* structure. For now, we only support setting TIDR for 'current' task.
*/
int set_thread_tidr(struct task_struct *t)
{
- int rc;
-
if (!cpu_has_feature(CPU_FTR_P9_TIDR))
return -EINVAL;
@@ -1602,11 +1511,7 @@ int set_thread_tidr(struct task_struct *t)
if (t->thread.tidr)
return 0;
- rc = assign_thread_tidr();
- if (rc < 0)
- return rc;
-
- t->thread.tidr = rc;
+ t->thread.tidr = (u16)task_pid_nr(t);
mtspr(SPRN_TIDR, t->thread.tidr);
return 0;
--
Andrew Donnellan OzLabs, ADL Canberra
andrew.donnel...@au1.ibm.com IBM Australia Limited
