Messages in this thread | | | Date | Fri, 26 Jun 2020 09:58:01 -0400 | From | Steven Rostedt <> | Subject | Re: [RFC][PATCH] ring-buffer: Have nested events still record running time stamp |
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On Thu, 25 Jun 2020 23:35:52 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
> I have to think about this more, as I think there's a flaw in this > cmpxchg algorithm.
Bah, that was all wrong. I need to incorporate the seq counter into the time stamp. Luckily I only use 59 bits for a full time stamp (gives me 18 years of nanoseconds), which gives me 5 bits to pay with, and all I need is 4 (2 for top and 2 for bottom). Now I can make the timestamp 60 bits with 30 bits from the top and 30 from the bottom, and using two bits in each of those as a sequence counter.
If after reading top and bottom, if the seq bits match, then the two match and can be put together as a valid number. Now I have this:
struct rb_time_struct { local_t cnt; local_t top; local_t bottom; };
typedef struct rb_time_struct rb_time_t; #define RB_TIME_SHIFT 30 #define RB_TIME_VAL_MASK ((1 << RB_TIME_SHIFT) - 1)
static int rb_time_cnt(unsigned long val) { return (val >> RB_TIME_SHIFT) & 3; }
static u64 rb_time_val(unsigned long top, unsigned long bottom) { u64 val;
val = top & RB_TIME_VAL_MASK; val <<= RB_TIME_SHIFT; val |= bottom & RB_TIME_VAL_MASK;
return val; }
static inline bool __rb_time_read(rb_time_t *t, u64 *ret, unsigned long *cnt) { unsigned long top, bottom;
top = local_read(&t->top); bottom = local_read(&t->bottom);
*cnt = rb_time_cnt(top);
if (*cnt != rb_time_cnt(bottom)) return false;
*ret = rb_time_val(top, bottom); return true; }
static bool rb_time_read(rb_time_t *t, u64 *ret) { unsigned long cnt;
return __rb_time_read(t, ret, &cnt); }
static inline unsigned long rb_time_val_cnt(unsigned long val, unsigned long cnt) { return (val & RB_TIME_VAL_MASK) | ((cnt & 3) << RB_TIME_SHIFT); }
static inline void rb_time_split(u64 val, unsigned long *top, unsigned long *bottom) { *top = (unsigned long)((val >> RB_TIME_SHIFT) & RB_TIME_VAL_MASK); *bottom = (unsigned long)(val & RB_TIME_VAL_MASK); }
static void rb_time_val_set(local_t *t, unsigned long val, unsigned long cnt) { val = rb_time_val_cnt(val, cnt); local_set(t, val); }
static void rb_time_set(rb_time_t *t, u64 val) { unsigned long cnt, top, bottom;
rb_time_split(val, &top, &bottom); do { cnt = local_inc_return(&t->cnt); rb_time_val_set(&t->top, top, cnt); rb_time_val_set(&t->bottom, bottom, cnt); } while (cnt != local_read(&t->cnt)); }
static bool rb_time_read_cmpxchg(local_t *l, unsigned long expect, unsigned long set) { unsigned long ret;
ret = local_cmpxchg(l, expect, set); return ret == expect; }
static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set) { unsigned long cnt, top, bottom; unsigned long cnt2, top2, bottom2; u64 val;
if (!__rb_time_read(t, &val, &cnt)) return false;
if (val != expect) return false;
cnt2 = local_read(&t->cnt); if (cnt2 != cnt) return false; cnt2++;
rb_time_split(val, &top, &bottom); top = rb_time_val_cnt(top, cnt); bottom = rb_time_val_cnt(bottom, cnt);
rb_time_split(set, &top2, &bottom2); top = rb_time_val_cnt(top2, cnt2); bottom = rb_time_val_cnt(bottom2, cnt2); if (!rb_time_read_cmpxchg(&t->cnt, cnt, cnt2)) return false; if (!rb_time_read_cmpxchg(&t->top, top, top2)) return false; if (!rb_time_read_cmpxchg(&t->bottom, bottom, bottom2)) return false; return true; }
Maybe I might implement this for BITS_FOR_LONG == 32.
-- Steve
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