Messages in this thread | | | Date | Wed, 29 Nov 2017 22:12:09 -0500 | From | Steven Rostedt <> | Subject | Re: [RESEND PATCH v3] tracing: Allocate mask_str buffer dynamically |
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On Wed, 29 Nov 2017 12:42:45 +0800 changbin.du@intel.com wrote:
> From: Changbin Du <changbin.du@intel.com> > > The default NR_CPUS can be very large, but actual possible nr_cpu_ids > usually is very small. For my x86 distribution, the NR_CPUS is 8192 and > nr_cpu_ids is 4. About 2 pages are wasted. > > Most machines don't have so many CPUs, so define a array with NR_CPUS > just wastes memory. So let's allocate the buffer dynamically when need. > > The exact buffer size should be: > DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2; > > Example output: > ff,ffffffff
Um, what if there's more than 64 CPUs, where I have booted several boxes that have more. There's going to be more than 1 comma.
> > With this change, the mutext tracing_cpumask_update_lock also can be > removed now, which was used to protect mask_str. > > Signed-off-by: Changbin Du <changbin.du@intel.com> > Cc: Steven Rostedt <rostedt@goodmis.org> > > --- > v3: > - remove tracing_cpumask_update_lock which was used to protect mask_str. (Rostedt) > v2: > - remove 'static' declaration. > - fix buffer size. > --- > kernel/trace/trace.c | 29 +++++++++-------------------- > 1 file changed, 9 insertions(+), 20 deletions(-) > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index 73e67b6..6750d05 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -4178,37 +4178,30 @@ static const struct file_operations show_traces_fops = { > .llseek = seq_lseek, > }; > > -/* > - * The tracer itself will not take this lock, but still we want > - * to provide a consistent cpumask to user-space: > - */ > -static DEFINE_MUTEX(tracing_cpumask_update_lock); > - > -/* > - * Temporary storage for the character representation of the > - * CPU bitmask (and one more byte for the newline): > - */ > -static char mask_str[NR_CPUS + 1]; > - > static ssize_t > tracing_cpumask_read(struct file *filp, char __user *ubuf, > size_t count, loff_t *ppos) > { > struct trace_array *tr = file_inode(filp)->i_private; > + char *mask_str; > int len; > > - mutex_lock(&tracing_cpumask_update_lock); > + /* Bitmap, ',' and two more bytes for the newline and '\0'. */ > + len = DIV_ROUND_UP(nr_cpu_ids, 4) + nr_cpu_ids/32 + 2;
This is broken. Instead do:
len = snprintf(NULL, 0, "%*pb\n", cpumask_pr_args(tr->tracing_cpumask)) + 1;
mask_str = kmalloc(len, GFP_KERNEL); [..] len = snprintf(mask_str, len, "%*pb\n", cpumask_pr_args(tr->tracing_cpumask));
-- Steve
> + mask_str = kmalloc(len, GFP_KERNEL); > + if (!mask_str) > + return -ENOMEM; > > - len = snprintf(mask_str, count, "%*pb\n", > + len = snprintf(mask_str, len, "%*pb\n", > cpumask_pr_args(tr->tracing_cpumask)); > if (len >= count) { > count = -EINVAL; > goto out_err; > } > - count = simple_read_from_buffer(ubuf, count, ppos, mask_str, NR_CPUS+1); > + count = simple_read_from_buffer(ubuf, count, ppos, mask_str, len); > > out_err: > - mutex_unlock(&tracing_cpumask_update_lock); > + kfree(mask_str); > > return count; > } > @@ -4228,8 +4221,6 @@ tracing_cpumask_write(struct file *filp, const char __user *ubuf, > if (err) > goto err_unlock; > > - mutex_lock(&tracing_cpumask_update_lock); > - > local_irq_disable(); > arch_spin_lock(&tr->max_lock); > for_each_tracing_cpu(cpu) { > @@ -4252,8 +4243,6 @@ tracing_cpumask_write(struct file *filp, const char __user *ubuf, > local_irq_enable(); > > cpumask_copy(tr->tracing_cpumask, tracing_cpumask_new); > - > - mutex_unlock(&tracing_cpumask_update_lock); > free_cpumask_var(tracing_cpumask_new); > > return count;
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