Messages in this thread | | | From | Kyle Moffett <> | Subject | Re: [Announce] Linux-tiny project revival | Date | Fri, 21 Sep 2007 17:34:05 -0400 |
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On Sep 20, 2007, at 19:18:41, Rob Landley wrote: > On Thursday 20 September 2007 4:26:13 pm Indan Zupancic wrote: >> But the problem remains that there are printk's which don't have a >> KERN_* as the first argument. Those are also impossible to get rid >> off in this way, as the loglevel is unknown (and you don't want >> partially printed messages). >> >> So adding the comma is really needed and in addition all printk's >> without a loglevel should get one. Which clutters the code and may >> increase codesize. > > It's ok to _explicitly_ not have a loglevel, and thus take a known > default. The problem is printing out less than a full line, > continuing it later, and not making obvious at compile time what > the level of this chunk is.
That's actually a fairly straightforward problem to solve. Really we ought to be able to guarantee that lines from different CPUs are not intermixed. This can end up in a log like this:
info: Current device state: <1>netdev watchdog timeout: eth0 reg1=0xABCD reg2=0xDEADBEEF
Clearly unpleasant, no? Really any logging which wants to print out a bunch of stuff with multiple printk()s should use an API something like this:
Option 1: Buffer allocated with kmalloc(). Sleep-ish context, can handle failures easily > struct printk_queue qpk; > if (qprintk_kmalloc(&qpk, level, GFP_KERNEL)) > return -ENOMEM; > > qprintk(&qpk, "Some string here:"); > qprintk(&qpk, " %d, %s, %d", 4, some_string, 42); > qprintk_finish(&qpk);
Option 2: Preallocated per-cpu buffer. preempt_disable()d > struct printk_queue qpk; > qprintk_percpu(&qpk, level); > [...] > qprintk_finish(&qpk);
Option 3: Preallocated per-cpu interrupts-only buffer. Only from code which may interrupt a preempt_disable() section > struct printk_queue qpk; > qprintk_irq(&pqk, level) > [...] > qprintk_finish(&qpk);
For all of the above, the final "qprintk_finish()" call would essentially take the printk spinlock and write the contents of the qpk->buffer into the dmesg ringbuffer, inserting ("<%d>", level) at the beginning and after each newline. The buffers would all be some useful fixed size (a page?); if you need more than that then you are probably trying to queue too much and excess data would be truncated.
Since the level would be a constant passed to qprintk_ {kmalloc,percpu,irq} in almost every case, you could easily do something like this:
static inline int qprintk_kmalloc(struct printk_queue *qpk, unsigned int level, gfp_t gfp) { if (level > CONFIG_MAX_LOG_LEVEL) { qpk->type = QPRINTK_TYPE_NONE; /* also 0 */ qpk->buffer = NULL; return 0; }
_qprintk_kmalloc(qpk, level, gfp); }
#define qprintk(QPK, FMT...) do { \ if ((QPK)->type) _qprintk(QPK, FMT); } while(0)
With a bit more glue that would cause GCC to notice that for a given qprintk_kmalloc the "qpk->type" is always zero because the level is too high, and therefore it would optimize out *ALL* of the _qprintk_kmalloc(), _qprintk(), and _qprintk_finish() calls.
Cheers, Kyle Moffett
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