[RFC][PATCH 0/2] PF_flags cleaups

[RFC][PATCH 0/2] PF_flags cleaups

[Peter Zijlstra]

Because we recently ran out of PF_flags, try and clean up.

Patches are on top of -tip, which already includes the PF_ALIGNWARN
removal.

[RFC][PATCH 1/2] kernel: remove PF_FLUSHER

[Peter Zijlstra]

[-- Attachment #1: pf-flags-1.patch --]
[-- Type: text/plain, Size: 1814 bytes --]

PF_FLUSHER is only ever set, not tested, remove it.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jens Axboe <axboe@kernel.dk>
---
 fs/fs-writeback.c     |    2 +-
 include/linux/sched.h |    1 -
 mm/backing-dev.c      |    2 +-
 3 files changed, 2 insertions(+), 3 deletions(-)

Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c
+++ linux-2.6/fs/fs-writeback.c
@@ -782,7 +782,7 @@ int bdi_writeback_thread(void *data)
 	struct backing_dev_info *bdi = wb->bdi;
 	long pages_written;
 
-	current->flags |= PF_FLUSHER | PF_SWAPWRITE;
+	current->flags |= PF_SWAPWRITE;
 	set_freezable();
 	wb->last_active = jiffies;
 
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1702,7 +1702,6 @@ extern void thread_group_times(struct ta
 #define PF_DUMPCORE	0x00000200	/* dumped core */
 #define PF_SIGNALED	0x00000400	/* killed by a signal */
 #define PF_MEMALLOC	0x00000800	/* Allocating memory */
-#define PF_FLUSHER	0x00001000	/* responsible for disk writeback */
 #define PF_USED_MATH	0x00002000	/* if unset the fpu must be initialized before use */
 #define PF_FREEZING	0x00004000	/* freeze in progress. do not account to load */
 #define PF_NOFREEZE	0x00008000	/* this thread should not be frozen */
Index: linux-2.6/mm/backing-dev.c
===================================================================
--- linux-2.6.orig/mm/backing-dev.c
+++ linux-2.6/mm/backing-dev.c
@@ -360,7 +360,7 @@ static int bdi_forker_thread(void *ptr)
 {
 	struct bdi_writeback *me = ptr;
 
-	current->flags |= PF_FLUSHER | PF_SWAPWRITE;
+	current->flags |= PF_SWAPWRITE;
 	set_freezable();
 
 	/*

[RFC][PATCH 2/2] kernel: extract thread types from task_struct::flags

[Peter Zijlstra]

[-- Attachment #1: pf-flags-2.patch --]
[-- Type: text/plain, Size: 14815 bytes --]

Free up a few more PF_flags by moving thread types out to their own variable.

Initially I compressed the types into less bits inside task_struct::flags, but
Thomas suggested I move them to their own field.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 fs/exec.c                   |    4 +-
 fs/xfs/linux-2.6/xfs_aops.c |    2 -
 include/linux/init_task.h   |    3 +
 include/linux/kvm_host.h    |    4 +-
 include/linux/sched.h       |   79 +++++++++++++++++++++++++++-----------------
 include/linux/swap.h        |    2 -
 kernel/exit.c               |    3 +
 kernel/fork.c               |   12 +++++-
 kernel/ptrace.c             |    2 -
 kernel/rtmutex-tester.c     |    2 -
 kernel/rtmutex_common.h     |    2 -
 kernel/sched.c              |    7 ++-
 kernel/workqueue.c          |    4 +-
 lib/is_single_threaded.c    |    2 -
 mm/oom_kill.c               |    4 +-
 mm/vmscan.c                 |    3 +
 16 files changed, 83 insertions(+), 52 deletions(-)

Index: linux-2.6/fs/exec.c
===================================================================
--- linux-2.6.orig/fs/exec.c
+++ linux-2.6/fs/exec.c
@@ -1401,7 +1401,7 @@ int do_execve(const char * filename,
 	if (retval < 0)
 		goto out;
 
-	current->flags &= ~PF_KTHREAD;
+	task_set_type(current, tt_user);
 	retval = search_binary_handler(bprm,regs);
 	if (retval < 0)
 		goto out;
@@ -1651,7 +1651,7 @@ static inline int zap_threads(struct tas
 	for_each_process(g) {
 		if (g == tsk->group_leader)
 			continue;
-		if (g->flags & PF_KTHREAD)
+		if (task_type(g) >= tt_kernel)
 			continue;
 		p = g;
 		do {
Index: linux-2.6/fs/xfs/linux-2.6/xfs_aops.c
===================================================================
--- linux-2.6.orig/fs/xfs/linux-2.6/xfs_aops.c
+++ linux-2.6/fs/xfs/linux-2.6/xfs_aops.c
@@ -1067,7 +1067,7 @@ xfs_vm_writepage(
 	 * filesystems like XFS, btrfs and ext4 have to take care of this
 	 * by themselves.
 	 */
-	if ((current->flags & (PF_MEMALLOC|PF_KSWAPD)) == PF_MEMALLOC)
+	if (task_type(current) != tt_kswapd && (current->flags & PF_MEMALLOC))
 		goto redirty;
 
 	/*
Index: linux-2.6/include/linux/init_task.h
===================================================================
--- linux-2.6.orig/include/linux/init_task.h
+++ linux-2.6/include/linux/init_task.h
@@ -117,7 +117,8 @@ extern struct cred init_cred;
 	.state		= 0,						\
 	.stack		= &init_thread_info,				\
 	.usage		= ATOMIC_INIT(2),				\
-	.flags		= PF_KTHREAD,					\
+	.flags		= 0,						\
+	.threadtype	= tt_kernel,					\
 	.lock_depth	= -1,						\
 	.prio		= MAX_PRIO-20,					\
 	.static_prio	= MAX_PRIO-20,					\
Index: linux-2.6/include/linux/kvm_host.h
===================================================================
--- linux-2.6.orig/include/linux/kvm_host.h
+++ linux-2.6/include/linux/kvm_host.h
@@ -509,13 +509,13 @@ static inline int kvm_deassign_device(st
 static inline void kvm_guest_enter(void)
 {
 	account_system_vtime(current);
-	current->flags |= PF_VCPU;
+	task_set_type(current, tt_vcpu);
 }
 
 static inline void kvm_guest_exit(void)
 {
 	account_system_vtime(current);
-	current->flags &= ~PF_VCPU;
+	task_set_type(current, tt_user);
 }
 
 static inline gpa_t gfn_to_gpa(gfn_t gfn)
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1168,12 +1168,26 @@ enum perf_event_task_context {
 	perf_nr_task_contexts,
 };
 
+/*
+ * Types >= tt_kernel imply the old PF_KTHREAD
+ */
+enum thread_type {
+	tt_user	= 0,		/* I'm a normal userspace thread */
+	tt_vcpu,		/* I'm a VCPU */
+	tt_kernel,		/* I'm a normal kernel thread */
+	tt_worker,		/* I'm a workqueue worker thread */
+	tt_kswapd,		/* I'm a kswap daemon thread */
+	tt_mutex_tester,	/* I'm a mutex tester thread */
+	tt_nr_max,
+};
+
 struct task_struct {
 	volatile long state;	/* -1 unrunnable, 0 runnable, >0 stopped */
 	void *stack;
 	atomic_t usage;
 	unsigned int flags;	/* per process flags, defined below */
 	unsigned int ptrace;
+	enum thread_type threadtype;
 
 	int lock_depth;		/* BKL lock depth */
 
@@ -1691,36 +1705,41 @@ extern void thread_group_times(struct ta
 /*
  * Per process flags
  */
-#define PF_STARTING	0x00000002	/* being created */
-#define PF_EXITING	0x00000004	/* getting shut down */
-#define PF_EXITPIDONE	0x00000008	/* pi exit done on shut down */
-#define PF_VCPU		0x00000010	/* I'm a virtual CPU */
-#define PF_WQ_WORKER	0x00000020	/* I'm a workqueue worker */
-#define PF_FORKNOEXEC	0x00000040	/* forked but didn't exec */
-#define PF_MCE_PROCESS  0x00000080      /* process policy on mce errors */
-#define PF_SUPERPRIV	0x00000100	/* used super-user privileges */
-#define PF_DUMPCORE	0x00000200	/* dumped core */
-#define PF_SIGNALED	0x00000400	/* killed by a signal */
-#define PF_MEMALLOC	0x00000800	/* Allocating memory */
-#define PF_USED_MATH	0x00002000	/* if unset the fpu must be initialized before use */
-#define PF_FREEZING	0x00004000	/* freeze in progress. do not account to load */
-#define PF_NOFREEZE	0x00008000	/* this thread should not be frozen */
-#define PF_FROZEN	0x00010000	/* frozen for system suspend */
-#define PF_FSTRANS	0x00020000	/* inside a filesystem transaction */
-#define PF_KSWAPD	0x00040000	/* I am kswapd */
-#define PF_OOM_ORIGIN	0x00080000	/* Allocating much memory to others */
-#define PF_LESS_THROTTLE 0x00100000	/* Throttle me less: I clean memory */
-#define PF_KTHREAD	0x00200000	/* I am a kernel thread */
-#define PF_RANDOMIZE	0x00400000	/* randomize virtual address space */
-#define PF_SWAPWRITE	0x00800000	/* Allowed to write to swap */
-#define PF_SPREAD_PAGE	0x01000000	/* Spread page cache over cpuset */
-#define PF_SPREAD_SLAB	0x02000000	/* Spread some slab caches over cpuset */
-#define PF_THREAD_BOUND	0x04000000	/* Thread bound to specific cpu */
-#define PF_MCE_EARLY    0x08000000      /* Early kill for mce process policy */
-#define PF_MEMPOLICY	0x10000000	/* Non-default NUMA mempolicy */
-#define PF_MUTEX_TESTER	0x20000000	/* Thread belongs to the rt mutex tester */
-#define PF_FREEZER_SKIP	0x40000000	/* Freezer should not count it as freezeable */
-#define PF_FREEZER_NOSIG 0x80000000	/* Freezer won't send signals to it */
+#define PF_STARTING	 0x00000001	/* being created */
+#define PF_EXITING	 0x00000002	/* getting shut down */
+#define PF_EXITPIDONE	 0x00000004	/* pi exit done on shut down */
+#define PF_FORKNOEXEC	 0x00000008	/* forked but didn't exec */
+#define PF_MCE_PROCESS   0x00000010	/* process policy on mce errors */
+#define PF_SUPERPRIV	 0x00000020	/* used super-user privileges */
+#define PF_DUMPCORE	 0x00000040	/* dumped core */
+#define PF_SIGNALED	 0x00000080	/* killed by a signal */
+#define PF_MEMALLOC	 0x00000100	/* Allocating memory */
+#define PF_USED_MATH	 0x00000200	/* if unset the fpu must be initialized before use */
+#define PF_FREEZING	 0x00000400	/* freeze in progress. do not account to load */
+#define PF_NOFREEZE	 0x00000800	/* this thread should not be frozen */
+#define PF_FROZEN	 0x00001000	/* frozen for system suspend */
+#define PF_FREEZER_SKIP	 0x00002000	/* Freezer should not count it as freezeable */
+#define PF_FREEZER_NOSIG 0x00004000	/* Freezer won't send signals to it */
+#define PF_FSTRANS	 0x00008000	/* inside a filesystem transaction */
+#define PF_OOM_ORIGIN	 0x00010000	/* Allocating much memory to others */
+#define PF_LESS_THROTTLE 0x00020000	/* Throttle me less: I clean memory */
+#define PF_RANDOMIZE	 0x00040000	/* randomize virtual address space */
+#define PF_SWAPWRITE	 0x00080000	/* Allowed to write to swap */
+#define PF_SPREAD_PAGE	 0x00100000	/* Spread page cache over cpuset */
+#define PF_SPREAD_SLAB	 0x00200000	/* Spread some slab caches over cpuset */
+#define PF_THREAD_BOUND	 0x00400000	/* Thread bound to specific cpu */
+#define PF_MCE_EARLY     0x00800000	/* Early kill for mce process policy */
+#define PF_MEMPOLICY	 0x01000000	/* Non-default NUMA mempolicy */
+
+static inline void task_set_type(struct task_struct *p, enum thread_type type)
+{
+	p->threadtype = type;
+}
+
+static inline enum thread_type task_type(struct task_struct *p)
+{
+	return p->threadtype;
+}
 
 /*
  * Only the _current_ task can read/write to tsk->flags, but other
Index: linux-2.6/include/linux/swap.h
===================================================================
--- linux-2.6.orig/include/linux/swap.h
+++ linux-2.6/include/linux/swap.h
@@ -23,7 +23,7 @@ struct bio;
 
 static inline int current_is_kswapd(void)
 {
-	return current->flags & PF_KSWAPD;
+	return task_type(current) == tt_kswapd;
 }
 
 /*
Index: linux-2.6/kernel/exit.c
===================================================================
--- linux-2.6.orig/kernel/exit.c
+++ linux-2.6/kernel/exit.c
@@ -431,7 +431,8 @@ void daemonize(const char *name, ...)
 	 * We don't want to have TIF_FREEZE set if the system-wide hibernation
 	 * or suspend transition begins right now.
 	 */
-	current->flags |= (PF_NOFREEZE | PF_KTHREAD);
+	task_set_type(current, tt_kernel);
+	current->flags |= PF_NOFREEZE;
 
 	if (current->nsproxy != &init_nsproxy) {
 		get_nsproxy(&init_nsproxy);
Index: linux-2.6/kernel/fork.c
===================================================================
--- linux-2.6.orig/kernel/fork.c
+++ linux-2.6/kernel/fork.c
@@ -570,7 +570,7 @@ struct mm_struct *get_task_mm(struct tas
 	task_lock(task);
 	mm = task->mm;
 	if (mm) {
-		if (task->flags & PF_KTHREAD)
+		if (task_type(task) >= tt_kernel)
 			mm = NULL;
 		else
 			atomic_inc(&mm->mm_users);
@@ -910,8 +910,16 @@ static int copy_signal(unsigned long clo
 static void copy_flags(unsigned long clone_flags, struct task_struct *p)
 {
 	unsigned long new_flags = p->flags;
+	unsigned long new_type;
 
-	new_flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);
+	if (task_type(p) >= tt_kernel)
+		new_type = tt_kernel;
+	else
+		new_type = tt_user;
+
+	task_set_type(p, new_type);
+
+	new_flags &= ~PF_SUPERPRIV;
 	new_flags |= PF_FORKNOEXEC;
 	new_flags |= PF_STARTING;
 	p->flags = new_flags;
Index: linux-2.6/kernel/ptrace.c
===================================================================
--- linux-2.6.orig/kernel/ptrace.c
+++ linux-2.6/kernel/ptrace.c
@@ -170,7 +170,7 @@ int ptrace_attach(struct task_struct *ta
 	audit_ptrace(task);
 
 	retval = -EPERM;
-	if (unlikely(task->flags & PF_KTHREAD))
+	if (unlikely(task_type(task) >= tt_kernel))
 		goto out;
 	if (same_thread_group(task, current))
 		goto out;
Index: linux-2.6/kernel/rtmutex-tester.c
===================================================================
--- linux-2.6.orig/kernel/rtmutex-tester.c
+++ linux-2.6/kernel/rtmutex-tester.c
@@ -259,7 +259,7 @@ static int test_func(void *data)
 	struct test_thread_data *td = data;
 	int ret;
 
-	current->flags |= PF_MUTEX_TESTER;
+	task_set_type(current, tt_mutex_tester);
 	set_freezable();
 	allow_signal(SIGHUP);
 
Index: linux-2.6/kernel/rtmutex_common.h
===================================================================
--- linux-2.6.orig/kernel/rtmutex_common.h
+++ linux-2.6/kernel/rtmutex_common.h
@@ -26,7 +26,7 @@ extern void schedule_rt_mutex_test(struc
 
 #define schedule_rt_mutex(_lock)				\
   do {								\
-	if (!(current->flags & PF_MUTEX_TESTER))		\
+	if (task_type(current) != tt_mutex_tester)		\
 		schedule();					\
 	else							\
 		schedule_rt_mutex_test(_lock);			\
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -2322,7 +2322,7 @@ static inline void ttwu_post_activation(
 	}
 #endif
 	/* if a worker is waking up, notify workqueue */
-	if ((p->flags & PF_WQ_WORKER) && success)
+	if (task_type(p) == tt_worker && success)
 		wq_worker_waking_up(p, cpu_of(rq));
 }
 
@@ -3380,7 +3380,8 @@ void account_system_time(struct task_str
 	struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
 	cputime64_t tmp;
 
-	if ((p->flags & PF_VCPU) && (irq_count() - hardirq_offset == 0)) {
+
+	if (task_type(p) == tt_vcpu && (irq_count() - hardirq_offset == 0)) {
 		account_guest_time(p, cputime, cputime_scaled);
 		return;
 	}
@@ -3772,7 +3773,7 @@ asmlinkage void __sched schedule(void)
 			 * task to maintain concurrency.  If so, wake
 			 * up the task.
 			 */
-			if (prev->flags & PF_WQ_WORKER) {
+			if (task_type(prev) == tt_worker) {
 				struct task_struct *to_wakeup;
 
 				to_wakeup = wq_worker_sleeping(prev, cpu);
Index: linux-2.6/kernel/workqueue.c
===================================================================
--- linux-2.6.orig/kernel/workqueue.c
+++ linux-2.6/kernel/workqueue.c
@@ -1888,14 +1888,14 @@ static int worker_thread(void *__worker)
 	struct global_cwq *gcwq = worker->gcwq;
 
 	/* tell the scheduler that this is a workqueue worker */
-	worker->task->flags |= PF_WQ_WORKER;
+	task_set_type(worker->task, tt_worker);
 woke_up:
 	spin_lock_irq(&gcwq->lock);
 
 	/* DIE can be set only while we're idle, checking here is enough */
 	if (worker->flags & WORKER_DIE) {
 		spin_unlock_irq(&gcwq->lock);
-		worker->task->flags &= ~PF_WQ_WORKER;
+		task_set_type(worker->task, tt_kernel);
 		return 0;
 	}
 
Index: linux-2.6/lib/is_single_threaded.c
===================================================================
--- linux-2.6.orig/lib/is_single_threaded.c
+++ linux-2.6/lib/is_single_threaded.c
@@ -31,7 +31,7 @@ bool current_is_single_threaded(void)
 	ret = false;
 	rcu_read_lock();
 	for_each_process(p) {
-		if (unlikely(p->flags & PF_KTHREAD))
+		if (unlikely(task_type(p) >= tt_kernel))
 			continue;
 		if (unlikely(p == task->group_leader))
 			continue;
Index: linux-2.6/mm/oom_kill.c
===================================================================
--- linux-2.6.orig/mm/oom_kill.c
+++ linux-2.6/mm/oom_kill.c
@@ -126,7 +126,7 @@ static bool oom_unkillable_task(struct t
 {
 	if (is_global_init(p))
 		return true;
-	if (p->flags & PF_KTHREAD)
+	if (task_type(p) >= tt_kernel)
 		return true;
 
 	/* When mem_cgroup_out_of_memory() and p is not member of the group */
@@ -356,7 +356,7 @@ static void dump_tasks(const struct mem_
 
 	pr_info("[ pid ]   uid  tgid total_vm      rss cpu oom_adj oom_score_adj name\n");
 	for_each_process(p) {
-		if (p->flags & PF_KTHREAD)
+		if (task_type(p) >= tt_kernel)
 			continue;
 		if (mem && !task_in_mem_cgroup(p, mem))
 			continue;
Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c
+++ linux-2.6/mm/vmscan.c
@@ -2316,7 +2316,8 @@ static int kswapd(void *p)
 	 * us from recursively trying to free more memory as we're
 	 * trying to free the first piece of memory in the first place).
 	 */
-	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
+	task_set_type(tsk, tt_kswapd);
+	tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE;
 	set_freezable();
 
 	order = 0;

Re: [RFC][PATCH 0/2] PF_flags cleaups

[Linus Torvalds]

On Mon, Sep 20, 2010 at 8:13 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> Because we recently ran out of PF_flags, try and clean up.
>
> Patches are on top of -tip, which already includes the PF_ALIGNWARN
> removal.

Looks ok by me conceptually, but I _really_ hate the naming of that
second patch and the pointless churn.

I also find it disgusting to have that [set_]task_type() "helper"
abstraction layer. It doesn't actually help anything at all, and just
makes it less clear what is going on.

So instead of code like this:

   if (task_type(p) >= tt_kernel)

why the heck isn't it just a much more readable old code (with just a
renamed new field) and just do

   if (task->task_type & TT_KTHREAD)

instead? Even if you were to want to use an enum rather than a set of
#define's, the all-caps thing makes it visually obvious that we're
talking about some constant rather than a variable named "tt_kernel".

I dunno. I'd personally also just keep it as a bitmask thing, if only
to make the patch more readable. IOW, the patch _should_ have just
turned

        tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;

into

        tsk->flags |= PF_MEMALLOC | PF_SWAPWRITE;
        tsk->task_type |= TT_KSWAPD;

and then the patch would always have been an "obvious straightforward
identity transform".

For example, look at the mess you did:


-       new_flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);
+       if (task_type(p) >= tt_kernel)
+               new_type = tt_kernel;
+       else
+               new_type = tt_user;
+
+       task_set_type(p, new_type);
+

and look how much straightforward it would have been had you just kept
the same simple semantics with just a new field:

-       new_flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);
+       new_type &= ~(TT_SUPERPRIV | TT_WQ_WORKER);

and nobody could possibly have any objections to a straightforward
"move the task type flags into a separate field" patch.

No?

                       Linus

Re: [RFC][PATCH 0/2] PF_flags cleaups

[Peter Zijlstra]

On Mon, 2010-09-20 at 08:40 -0700, Linus Torvalds wrote:
> On Mon, Sep 20, 2010 at 8:13 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > Because we recently ran out of PF_flags, try and clean up.
> >
> > Patches are on top of -tip, which already includes the PF_ALIGNWARN
> > removal.
> 
> Looks ok by me conceptually, but I _really_ hate the naming of that
> second patch and the pointless churn.
> 

> and look how much straightforward it would have been had you just kept
> the same simple semantics with just a new field:
> 
> -       new_flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER);
> +       new_type &= ~(TT_SUPERPRIV | TT_WQ_WORKER);
> 
> and nobody could possibly have any objections to a straightforward
> "move the task type flags into a separate field" patch.

Sure, can do. Like said, my initial approach was to compress these type
bits into fewer bits by converting all these individual bits (PF_KSWAPD,
PF_WQ_WORKER, etc) into a linear range which spans less bits.

But indeed, if we're OK with adding a new field (which is I think the
biggest question, and your reply seems imply you don't mind at all),
then keeping the old structure and moving them over to a new field will
generate a much saner patch.

(I only left the helper functions in in case people would object to
adding another field and we'd need to really compress bits again).

One point though, I noticed we actually expose p->flags to userspace,
which basically makes PF_flags an ABI, do we care?

Re: [RFC][PATCH 0/2] PF_flags cleaups

[Linus Torvalds]

On Mon, Sep 20, 2010 at 8:58 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> One point though, I noticed we actually expose p->flags to userspace,
> which basically makes PF_flags an ABI, do we care?

Hmm. Where? And no, I don't think we care. But let's keep it in mind
if something subtle breaks in surprising ways.

                                Linus

Re: [RFC][PATCH 1/2] kernel: remove PF_FLUSHER

[Jens Axboe]

On 2010-09-20 17:13, Peter Zijlstra wrote:
> PF_FLUSHER is only ever set, not tested, remove it.

Looks good, it probably didn't get dropped appropriately
when current_is_pdflush() was killed.

-- 
Jens Axboe

Re: [RFC][PATCH 0/2] PF_flags cleaups

[Peter Zijlstra]

On Mon, 2010-09-20 at 09:15 -0700, Linus Torvalds wrote:
> On Mon, Sep 20, 2010 at 8:58 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > One point though, I noticed we actually expose p->flags to userspace,
> > which basically makes PF_flags an ABI, do we care?
> 
> Hmm. Where? And no, I don't think we care. But let's keep it in mind
> if something subtle breaks in surprising ways.

fs/proc/array.c:do_task_stat()

I'll rework the patches and simply move the bits around to another
field. We'll see what falls over.. :-)

Re: [RFC][PATCH 0/2] PF_flags cleaups

[Linus Torvalds]

On Mon, Sep 20, 2010 at 11:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> fs/proc/array.c:do_task_stat()

Ok, I doubt anybody cares. I think that thing is entirely historical,
and we probably should never have done it in the first place. I wonder
if we should just replace that field with a zero.

We've moved flag bits around in task->flags (and moved them to
thread->flags) before, so that /proc/<pid>/stat field has not been
stable historically either. I can't imagine what could break - it
would have broken many times before too.

                      Linus

Re: [RFC][PATCH 2/2] kernel: extract thread types from task_struct::flags

[Andrew Morton]

On Mon, 20 Sep 2010 17:13:36 +0200
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> Free up a few more PF_flags by moving thread types out to their own variable.
> 
> Initially I compressed the types into less bits inside task_struct::flags, but
> Thomas suggested I move them to their own field.
> 

There doesn't seem to be a huge point to all this, but I guess there's
some sense in separating "what type of thread this is" from "attributes
of this thread".  Maybe.  At the expense of a larger task_struct.

> +/*
> + * Types >= tt_kernel imply the old PF_KTHREAD
> + */

Perhaps that should be encapsulated into another helper function

> --- linux-2.6.orig/kernel/fork.c
> +++ linux-2.6/kernel/fork.c
> @@ -570,7 +570,7 @@ struct mm_struct *get_task_mm(struct tas
>  	task_lock(task);
>  	mm = task->mm;
>  	if (mm) {
> -		if (task->flags & PF_KTHREAD)
> +		if (task_type(task) >= tt_kernel)

rather than open-coded everywhere.

Re: [RFC][PATCH 2/2] kernel: extract thread types from task_struct::flags

[Peter Zijlstra]

On Mon, 2010-09-20 at 12:14 -0700, Andrew Morton wrote:
> On Mon, 20 Sep 2010 17:13:36 +0200
> Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > Free up a few more PF_flags by moving thread types out to their own variable.
> > 
> > Initially I compressed the types into less bits inside task_struct::flags, but
> > Thomas suggested I move them to their own field.
> > 
> 
> There doesn't seem to be a huge point to all this, but I guess there's
> some sense in separating "what type of thread this is" from "attributes
> of this thread".  Maybe.  At the expense of a larger task_struct. 

Yeah, with the PF_FLUSHER and PF_ALIGNWARN things removed we again have
2 bits free in flags.

And that's the amount -rt consumes (it adds 2 more: I'm a $FOO thread,
flags).

But having looked at it, I came up with the compress I'm a $FOO thread,
idea. Which would free up a few more bits for future use.

At that time Thomas suggested I move it into a separate field, and then
Linus said, don't compact the types, simply transfer the bits we have.

Anyway, it was all very much RFC.. and comments I got :-)

So can you agree with Linus' proposal of simply moving all type like
bits over to a new word, or would you rather see the linear type field
with an extra helper; possibly part of the existing ->flags field?

Re: [RFC][PATCH 2/2] kernel: extract thread types from task_struct::flags

[Andrew Morton]

On Mon, 20 Sep 2010 21:25:16 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> So can you agree with Linus' proposal of simply moving all type like
> bits over to a new word, or would you rather see the linear type field
> with an extra helper; possibly part of the existing ->flags field?

No strong feelings either way, really.

One thing we should keep an eye out for is the size/speed of code at
the client callsites.  Plucking a field of bits out of the flags word
and then treating that as a scalar would generate quite a bit of code,
I expect.  Let's not do that.