内核event trace时使用过滤器无法设置字符串匹配

Question

/sys/kernel/debug/tracing/events/rcu/rcu_utilization 下打开对应的 event trace 开关后有收集到以下信息：

   ktimersoftd/3-44    [003] d...1..  3441.221232: rcu_utilization: Start context switch
   ktimersoftd/3-44    [003] d...1..  3441.221233: rcu_utilization: End context switch
   ktimersoftd/3-44    [003] ....2..  3441.221233: rcu_utilization: Start context switch
   ktimersoftd/3-44    [003] ....2..  3441.221233: rcu_utilization: End context switch
          <idle>-0     [003] .N..2..  3441.222172: rcu_utilization: Start context switch
          <idle>-0     [003] .N..2..  3441.222173: rcu_utilization: End context switch
      cu_pr_gtpu-4883  [003] d..h1..  3441.222178: rcu_utilization: Start scheduler-tick
      cu_pr_gtpu-4883  [003] d..h1..  3441.222178: rcu_utilization: End scheduler-tick
      cu_pr_gtpu-4883  [003] ....2..  3441.222187: rcu_utilization: Start context switch

sh-4.2# ls
enable  filter  format  id
sh-4.2# pwd
/sys/kernel/debug/tracing/events/rcu/rcu_utilization

在上述目录下查看 format 内容格式，然后往过滤器中设置字符串，但提示错误如下：

sh-4.2# cat format
name: rcu_utilization
ID: 219
format:
        field:unsigned short common_type;       offset:0;       size:2; signed:0;
        field:unsigned char common_flags;       offset:2;       size:1; signed:0;
        field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
        field:int common_pid;   offset:4;       size:4; signed:1;
        field:unsigned short common_migrate_disable;    offset:8;       size:2; signed:0;
        field:unsigned short common_padding;    offset:10;      size:2; signed:0;

        field:const char * s;   offset:16;      size:8; signed:0;

print fmt: "%s", REC->s
sh-4.2#echo "s == End context switch" > filter
sh: echo: write error: Invalid argument
sh-4.2# cat filter
s == End context switch
^
parse_error: Illegal integer value